[media] media: rcar-vin: allow field to be changed

The driver forced whatever field was set by the source subdevice to be
used. This patch allows the user to change from the default field.

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>