Osram lightify Removed wrong assignment (#7615)

self._brightness is assigned with the returned value of the
set_luminance() function, which is always equal to None.

homeassistant/components/light/osramlightify.py

@@ -195,7 +195,7 @@ class Luminary(Light):
             self._brightness = kwargs[ATTR_BRIGHTNESS]
             _LOGGER.debug("turn_on requested brightness for light: %s is: %s ",
                           self._name, self._brightness)
-            self._brightness = self._luminary.set_luminance(
+            self._luminary.set_luminance(
                 int(self._brightness / 2.55),
                 transition)