devices: fix 'is_rssi' property of LinkQuality

Signed-off-by: Tatiana Leon <Tatiana.Leon@digi.com>

Author: tatianaleon

digi/xbee/devices.py

@@ -11949,7 +11949,7 @@ def is_rssi(self):
         Returns:
              Boolean: `True` if this is an RSSI value, `False` for LQI.
         """
-        return self.__lq
+        return self.__is_rssi
 
 
 LinkQuality.UNKNOWN = LinkQuality(lq=LinkQuality.UNKNOWN_VALUE)