Update brightness for Matrix featherwing

Author: makermelissa

adafruit_featherwing/matrix_featherwing.py

@@ -174,8 +174,10 @@ def brightness(self):
         Brightness returns the current display brightness.
         0-15 = Dimmest to Brightest Setting
         """
-        return self._matrix.brightness
+        return round(self._segments.brightness * 15)
 
     @brightness.setter
     def brightness(self, brightness):
-        self._matrix.brightness = brightness
+        if not 0 <= brightness <= 15:
+            raise ValueError('Brightness must be a value between 0 and 15')
+        self._segments.brightness = brightness / 15