Allow negative tx power

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

examples/simple_secure_chat/main.cpp

@@ -66,7 +66,7 @@ struct NodePrefs {  // persisted to file
   char node_name[32];
   double node_lat, node_lon;
   float freq;
-  uint8_t tx_power_dbm;
+  int8_t tx_power_dbm;
   uint8_t unused[3];
 };
 
@@ -290,7 +290,7 @@ public:
   }
 
   float getFreqPref() const { return _prefs.freq; }
-  uint8_t getTxPowerPref() const { return _prefs.tx_power_dbm; }
+  int8_t getTxPowerPref() const { return _prefs.tx_power_dbm; }
 
   void begin(FILESYSTEM& fs) {
     _fs = &fs;