media: rcar-vin: fix wrong return value in rvin_set_channel_routing()

If the operation in rvin_set_channel_routing() is successful the 'ret'
variable contains the runtime PM use count for the VIN master device.
The intention is not to return the use count to the caller but to return
0 on success else none zero.

Fix this by always returning 0 if the operation is successful.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>