video: Adjust video_clear() to return an error

All driver-model operation should return an error code. Adjust this
function to do so also.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Anatolij Gustschin <agust@denx.de>
diff --git a/include/video.h b/include/video.h
index cd5558f..ab416c1 100644
--- a/include/video.h
+++ b/include/video.h
@@ -120,8 +120,9 @@
  * video_clear() - Clear a device's frame buffer to background color.
  *
  * @dev:	Device to clear
+ * @return 0
  */
-void video_clear(struct udevice *dev);
+int video_clear(struct udevice *dev);
 
 /**
  * video_sync() - Sync a device's frame buffer with its hardware