[Solax X1] Optimize serial receive

[SteWers]

Description:

Serial receive - more stable, less complex and less code

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.2.7.8
• The code change is tested and works with Tasmota core ESP32 V.3.1.0.241030
• I accept the CLA.