Both radio-keene and dsbr100 did one v4l2_device_get too many. Thus the refcount
never became 0 and that causes a memory leak.

Also updated the V4L2 framework documentation accordingly.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>