media: rcar-vin: Add check for completed capture before completing buffer

Before reading which slot was captured to by examining the module status
(VnMS) register, make sure something was captured at all by examining
the interrupt status register (VnINTS).

Failing this a buffer maybe completed before it was captured too.

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