[media] cx18: Fix bytes_per_line

Current GStreamer userspace respects the bytes_per_line from the driver. Set
it to something reasonable for the format chosen.

Signed-off-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>