media: uvcvideo: Remove void casting for the status endpoint

Make the code more resilient, by replacing the castings with proper
structure definitions and using offsetof() instead of open coding the
location of the data.

Suggested-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Reviewed-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>