feat(RadioInterface): Tx power gain calculation rework (#6796)

- Rename REGULATORY_GAIN_LORA to TX_GAIN_LORA
- Move gain-based Tx power clamping from RadioInterface::applyModemConfig() to RadioInterface::limitPower()
  - User-configured Tx power now matches the Tx power out of the device connector
- Re-order [LoRa Chip]Interface.cpp limitPower() to take place before the final Tx power clamping so we clamp based on the pre-PA Tx power rather than user-configured Tx power

Tested on XIAO BLE variant.

Signed-off-by: Andrew Yong <me@ndoo.sg>
Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

src/mesh/SX126xInterface.cpp

@@ -69,11 +69,11 @@ template <typename T> bool SX126xInterface<T>::init()
 
     RadioLibInterface::init();
 
+    limitPower();
+
     if (power > SX126X_MAX_POWER) // Clamp power to maximum defined level
         power = SX126X_MAX_POWER;
 
-    limitPower();
-
     int res = lora.begin(getFreq(), bw, sf, cr, syncWord, power, preambleLength, tcxoVoltage, useRegulatorLDO);
     // \todo Display actual typename of the adapter, not just `SX126x`
     LOG_INFO("SX126x init result %d", res);