Add MAX_LORA_TX_POWER build flag for Heltec V4 configuration

variants/heltec_v4/platformio.ini

@@ -26,6 +26,7 @@ build_flags =
   -D PIN_VEXT_EN=36
   -D PIN_VEXT_EN_ACTIVE=HIGH
   -D LORA_TX_POWER=10 ;If it is configured as 10 here, the final output will be 22 dbm.
+  -D MAX_LORA_TX_POWER=22 ; Max SX1262 output
   -D SX126X_DIO2_AS_RF_SWITCH=true
   -D SX126X_DIO3_TCXO_VOLTAGE=1.8
   -D SX126X_CURRENT_LIMIT=140