[media] media: rcar-vin: fix height for TOP and BOTTOM fields

The height used for V4L2_FIELD_TOP and V4L2_FIELD_BOTTOM where wrong.
The frames only contain one field so the height should be half of the
frame.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>