[smartmeter] Thinghandler spams logs if serial port is unreachable

[t2000]

Currently the ThingHandler of the smartmeter binding floods the logs with warnings every 2 seconds if it cannot open the serial port:

2021-01-11 18:55:01.005 [WARN ] [ding.smartmeter.internal.MeterDevice] - Failed to read: Serial port /dev/ttyUSB0 not found or port is busy.. Closing connection and trying again in 2 seconds...; smartmeter:meter:heatingEnergy
org.openmuc.jrxtx.PortNotFoundException: Serial port /dev/ttyUSB0 not found or port is busy.
        at org.openmuc.jrxtx.JRxTxPort.openSerialPort(JRxTxPort.java:64) ~[bundleFile:?]
        at org.openmuc.jrxtx.SerialPortBuilder.build(SerialPortBuilder.java:140) ~[bundleFile:?]
        at org.openmuc.j62056.Iec21Port.<init>(Iec21Port.java:270) ~[bundleFile:?]
        at org.openmuc.j62056.Iec21Port.<init>(Iec21Port.java:47) ~[bundleFile:?]
        at org.openmuc.j62056.Iec21Port$Builder.buildAndOpen(Iec21Port.java:243) ~[bundleFile:?]
        at org.openhab.binding.smartmeter.internal.iec62056.Iec62056_21SerialConnector.openConnection(Iec62056_21SerialConnector.java:126) ~[bundleFile:?]
        at org.openhab.binding.smartmeter.internal.MeterDevice.lambda$1(MeterDevice.java:168) ~[bundleFile:?]
        at io.reactivex.internal.operators.flowable.FlowableDoOnLifecycle$SubscriptionLambdaSubscriber.onSubscribe(FlowableDoOnLifecycle.java:63) [bundleFile:?]
        at io.reactivex.internal.operators.flowable.FlowableTimeoutTimed.subscribeActual(FlowableTimeoutTimed.java:47) [bundleFile:?]
        at io.reactivex.Flowable.subscribe(Flowable.java:14479) [bundleFile:?]
        at io.reactivex.internal.operators.flowable.FlowableDoOnLifecycle.subscribeActual(FlowableDoOnLifecycle.java:38) [bundleFile:?]
        at io.reactivex.Flowable.subscribe(Flowable.java:14479) [bundleFile:?]
        at io.reactivex.internal.operators.flowable.FlowableDoOnEach.subscribeActual(FlowableDoOnEach.java:50) [bundleFile:?]
        at io.reactivex.Flowable.subscribe(Flowable.java:14479) [bundleFile:?]
        at io.reactivex.internal.operators.flowable.FlowableDoOnLifecycle.subscribeActual(FlowableDoOnLifecycle.java:38) [bundleFile:?]
        at io.reactivex.Flowable.subscribe(Flowable.java:14479) [bundleFile:?]
        at io.reactivex.internal.operators.flowable.FlowableDoOnEach.subscribeActual(FlowableDoOnEach.java:50) [bundleFile:?]
        at io.reactivex.Flowable.subscribe(Flowable.java:14479) [bundleFile:?]
        at io.reactivex.internal.operators.flowable.FlowablePublish.connect(FlowablePublish.java:130) [bundleFile:?]
        at io.reactivex.internal.operators.flowable.FlowableRefCount.subscribeActual(FlowableRefCount.java:91) [bundleFile:?]
        at io.reactivex.Flowable.subscribe(Flowable.java:14479) [bundleFile:?]
        at io.reactivex.Flowable.subscribe(Flowable.java:14426) [bundleFile:?]
        at io.reactivex.internal.operators.flowable.FlowableRepeatWhen$WhenReceiver.onNext(FlowableRepeatWhen.java:100) [bundleFile:?]
        at io.reactivex.subscribers.SerializedSubscriber.onNext(SerializedSubscriber.java:100) [bundleFile:?]
        at io.reactivex.internal.operators.flowable.FlowableDelay$DelaySubscriber$OnNext.run(FlowableDelay.java:114) [bundleFile:?]
        at io.reactivex.internal.schedulers.ExecutorScheduler$ExecutorWorker$BooleanRunnable.run(ExecutorScheduler.java:260) [bundleFile:?]
        at io.reactivex.internal.schedulers.ExecutorScheduler$ExecutorWorker.run(ExecutorScheduler.java:225) [bundleFile:?]
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515) [?:?]
        at java.util.concurrent.FutureTask.run(FutureTask.java:264) [?:?]
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304) [?:?]
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) [?:?]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) [?:?]
        at java.lang.Thread.run(Thread.java:834) [?:?]

However, it correctly sets the ThingStatus to OFFLINE (COMMUNICATION_ERROR): Serial port /dev/energyheat not found or port is busy. and retries again, so the warnings and the information about the retry should be set to DEBUG level. The user sees the issue in the ThingStatus can can fix it without needing these warnings in the logs.

I will provide a PR for this.