Allow negative tx power

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

variants/heltec_v2/target.cpp

@@ -43,7 +43,7 @@ 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);
 }