Allow negative tx power

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

examples/simple_room_server/MyMesh.h

@@ -188,7 +188,7 @@ public:
   }
 
   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");