[media] rcar-vin: fix bug in pixelformat selection

If the requested pixelformat is not supported fallback to the default
format, do not revert the entire format.

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>