Allow negative tx power

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

examples/simple_sensor/SensorMesh.h

@@ -66,7 +66,7 @@ public:
   void setLoggingOn(bool enable) override {  }
   void eraseLogFile() override { }
   void dumpLogFile() override { }
-  void setTxPower(uint8_t power_dbm) override;
+  void setTxPower(int8_t power_dbm) override;
   void formatNeighborsReply(char *reply) override {
     strcpy(reply, "not supported");
   }