[media] cx18: embed video_device

Embed the video_device struct to simplify the error handling and in
order to (eventually) get rid of video_device_alloc/release.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>