Add unique_id for decora_wifi lights (#66142)

I have some decora_wifi switches that I want to be able to move around
to different rooms within Home Assistant, and for that to be practical,
they need unique IDs, so here we are.

Signed-off-by: Jarod Wilson <jarodwilson@gmail.com>

homeassistant/components/decora_wifi/light.py

@@ -96,6 +96,7 @@ class DecoraWifiLight(LightEntity):
     def __init__(self, switch):
         """Initialize the switch."""
         self._switch = switch
+        self._attr_unique_id = switch.serial
 
     @property
     def supported_features(self):