[media] cx231xx: zero priv field and use right width in try_fmt

The priv field of v4l2_pix_format must be zeroed. Also fix a bug in try_fmt
where the current width was used instead of the width passed to try_fmt.

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