don't accept 0xFF as XBee Wifi strength

src/TinyGsmClientXBee.h

@@ -657,6 +657,8 @@ public:
     XBEE_COMMAND_END_DECORATOR
 
     if (beeType == XBEE3_LTEM_ATT && intRes == 105) intRes = 0;  // tends to reply with "69" when signal is unknown
+    if (beeType == XBEE3_LTEM_ATT && intRes == 0xFF) intRes = 0;  // 0xFF returned for unknown
+
     if (beeType == XBEE_S6B_WIFI) return -93 + intRes;  // the maximum sensitivity is -93dBm
     else return -1*intRes; // need to convert to negative number
   }