Allow negative tx power

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

variants/tiny_relay/target.cpp

@@ -70,7 +70,7 @@ 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);
 }