[media] rcar-vin: fix standard in input enumeration

The driver supports a single input only, which can be either analog or
digital. If the subdevice supports dv_timings_cap the input is digital
and the driver should not fill in the standard.

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