Implement setting TX_GAIN_LORA for portduino

[Stary2001]

I've implemented support for TX_GAIN_LORA in the portduino yaml config. Also refactors the code a little bit to support choosing linear/nonlinear gain at runtime for this.
Tested with this config:

Lora:
 Module: sx1262
 DIO2_AS_RF_SWITCH: true
 DIO3_TCXO_VOLTAGE: true
 CS: 8
 IRQ: 27
 Busy: 22
 Reset: 23
 TX_GAIN_LORA: 20
 SX1262_MAX_POWER: 12

🤝 Attestations

• I have tested that my proposed changes behave as described.
• I have tested that my proposed changes do not cause any obvious regressions on the following devices:
  • native
  • T1000E