Make brightness display work for rgb devices. (#5675)

* Make brightness display work for rgb devices.

Non rgbw devices return 255 for getWarmWhite255. This is part 2 to make the brightness slider work for these devices.

https://github.com/Danielhiversen/flux_led/pull/25

* Query brightness property which return WW level or RGB brightness equivalent.

https://github.com/Danielhiversen/flux_led/pull/25

homeassistant/components/light/flux_led.py

@@ -121,7 +121,7 @@ class FluxLight(Light):
     @property
     def brightness(self):
         """Return the brightness of this light between 0..255."""
-        return self._bulb.getWarmWhite255()
+        return self._bulb.brightness
 
     @property
     def rgb_color(self):