Allow negative tx power

Like SX1262 allows -9 dBm lowest, some allow lower but that probably
isn't useful

examples/companion_radio/MyMesh.cpp

@@ -838,7 +838,7 @@ void MyMesh::begin(bool has_display) {
   _prefs.bw = constrain(_prefs.bw, 7.8f, 500.0f);
   _prefs.sf = constrain(_prefs.sf, 5, 12);
   _prefs.cr = constrain(_prefs.cr, 5, 8);
-  _prefs.tx_power_dbm = constrain(_prefs.tx_power_dbm, 1, MAX_LORA_TX_POWER);
+  _prefs.tx_power_dbm = constrain(_prefs.tx_power_dbm, -9, MAX_LORA_TX_POWER);
   _prefs.gps_enabled = constrain(_prefs.gps_enabled, 0, 1);  // Ensure boolean 0 or 1
   _prefs.gps_interval = constrain(_prefs.gps_interval, 0, 86400);  // Max 24 hours
 

examples/companion_radio/NodePrefs.h

@@ -17,7 +17,7 @@ struct NodePrefs {  // persisted to file
   uint8_t multi_acks;
   uint8_t manual_add_contacts;
   float bw;
-  uint8_t tx_power_dbm;
+  int8_t tx_power_dbm;
   uint8_t telemetry_mode_base;
   uint8_t telemetry_mode_loc;
   uint8_t telemetry_mode_env;