[SX126x/SX128x] Start reading from Rx buffer offset

[GUVWAF]

As discussed in #1159 and similar to #1184, this will let SX126x and SX128x radios read the data in its buffer from the offset as returned by GetRxBufferStatus in order to match the data with the returned packet length of the last packet in case you skipped reading a packet. The behaviour is the same for both radios and this is the relevant part of the datasheet:

Two possibilities exist to obtain the offset value:
•First is to use the RxBaseAddr value since the user defines it before receiving a payload.
•Second, offset can be initialized with the value of RxStartBufferPointer returned by GetRxbufferStatus(...) command.
Note:
(...) When receiving, if the packet size exceeds the buffer memory allocated for the Rx, it will overwrite the
transmit portion of the data buffer.

So we're now changing to the second method, with the downside that there's a higher chance you will overwrite Rx data when transmitting. However, I would argue that it's easier to make sure you don't transmit before reading available data than to make sure you always read the data in time.

For the SX126x, I had to remove the call to setBufferBaseAddress(), since it will force the data to start at 0, while the returned pointer is still from the previous packet (if you don't go to standby in between). I tested that this works with and without going to standby in between.

For the SX128x, there was no call to setBufferBaseAddress(), but it calls standBy() in readData() and therefore the returned pointer will be correct. I'm not sure if it's really necessary anymore to call standBy() here (and thus the interrupt example wouldn't need to call startReceive() again), but since I don't have the hardware, I haven't tested this.

[jgromes]

Thank you for this fix! I left two minor comments, after those are addressed this can be merged.