Add a second release function: video_device_release_empty
It can be used by drivers that have statically allocated
video_device structs.

Its use usually, but not always, indicates laziness on the
part of the driver programmer.
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>