[media] rcar-vin: fix how pads are handled for v4l2 subdevice operations

The rcar-vin driver only uses one pad, pad number 0.

- All v4l2 operations that did not check that the requested operation
  was for pad 0 have been updated with a check to enforce this.

- All v4l2 operations that stored (and later restored) the requested pad
  before substituting it for the subdevice pad number have been updated
  to not store the incoming pad and simply restore it to 0 after the
  subdevice operation is complete.

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>