Allow negative tx power

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

variants/generic_espnow/target.cpp

@@ -25,7 +25,7 @@ void radio_set_params(float freq, float bw, uint8_t sf, uint8_t cr) {
   // no-op
 }
 
-void radio_set_tx_power(uint8_t dbm) {
+void radio_set_tx_power(int8_t dbm) {
   radio_driver.setTxPower(dbm);
 }