Allow negative tx power

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

variants/heltec_tracker/target.h

@@ -43,5 +43,5 @@ extern HWTSensorManager 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();