Allow negative tx power

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

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;