[media] usbvision: 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>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>