Allow negative tx power

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

examples/simple_repeater/MyMesh.cpp

@@ -899,7 +899,7 @@ void MyMesh::dumpLogFile() {
   }
 }
 
-void MyMesh::setTxPower(uint8_t power_dbm) {
+void MyMesh::setTxPower(int8_t power_dbm) {
   radio_set_tx_power(power_dbm);
 }