V4L/DVB: v4l: videobuf: code cleanup

Make videobuf pass checkpatch; minor code cleanups.

Signed-off-by: Pawel Osciak <p.osciak@samsung.com>
Reviewed-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>