Fix battery entry in device

Enforce use of enum value.

Fixes pwr-Solaar#2700
Related pwr-Solaar#2273

lib/logitech_receiver/device.py

@@ -393,7 +393,7 @@ def battery(self):  # None  or  level, next, status, voltage
                 try:
                     feature, battery = result
                     if self.persister and battery_feature is None:
-                        self.persister["_battery"] = feature
+                        self.persister["_battery"] = feature.value
                     return battery
                 except Exception:
                     if self.persister and battery_feature is None: