Shelly Dimmer: Use global setting value for brightness and fade rate at startup

[jamesturton]

Description:

Use global setting value for brightness and fade rate at startup. This way the Shelly Dimmer obeys PowerOnState.
Related issue (if applicable): fixes #10154

Checklist:

• The pull request is done against the latest dev branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works on Tasmota core ESP8266 V.2.7.4.9
• The code change is tested and works on Tasmota core ESP32 V.1.0.4.2
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass

[arendst]

Thx!