Allow negative tx power

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

examples/simple_repeater/MyMesh.h

@@ -198,7 +198,7 @@ public:
   }
 
   void dumpLogFile() override;
-  void setTxPower(uint8_t power_dbm) override;
+  void setTxPower(int8_t power_dbm) override;
   void formatNeighborsReply(char *reply) override;
   void removeNeighbor(const uint8_t* pubkey, int key_len) override;
   void formatStatsReply(char *reply) override;