Allow negative tx power

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

variants/thinknode_m2/target.cpp

@@ -46,7 +46,7 @@ void radio_set_params(float freq, float bw, uint8_t sf, uint8_t cr) {
   radio.setCodingRate(cr);
 }
 
-void radio_set_tx_power(uint8_t dbm) {
+void radio_set_tx_power(int8_t dbm) {
   radio.setOutputPower(dbm);
 }
 

variants/thinknode_m2/target.h

@@ -26,7 +26,7 @@ extern SensorManager sensors;
 bool radio_init();
 uint32_t radio_get_rng_seed();
 void radio_set_params(float freq, float bw, uint8_t sf, uint8_t cr);
-void radio_set_tx_power(uint8_t dbm);
+void radio_set_tx_power(int8_t dbm);
 mesh::LocalIdentity radio_new_identity();