media: rcar-vin: Limit NV12 availability to supported VIN channels only

When adding support for NV12 it was overlooked that the pixel format is
only supported on some VIN channels. Fix this by adding a check to only
accept NV12 on the supported channels 0, 1, 4, 5, 8, 9, 12 and 13.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>