Baudrate and serial config for modbus_bridge are now persistent

[jeroenst]

Description:

The baudrate and serial config where not saved when configuring. Also requesting the baudrate didn't return the correct baudrate. Baudrate and serial config are now saved and checked if valid before applied to serial port.

Related issue (if applicable): fixes #9586 (comment)

Checklist:

• The pull request is done against the latest development 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 with Tasmota core ESP8266 V.2.7.4.9
• The code change is tested and works with Tasmota core ESP32 V.2.0.4 (N/A for TasmotaModbus)
• I accept the CLA.

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