[media] cx25821: embed video_device, clean up some kernel log spam

Embed the video_device struct instead of allocating it.
Remove some of the annoying and ugly kernel messages shown during
loading and unloading of the module.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>