media: rcar-vin: fix return, use ret instead of zero

Currently the return error code is in ret is being assigned but not
used. It and should be returned by the return statement and currently
just 0 is being returned. Fix this.

Addresses-Coverity: ("Unused value")

Fixes: b9ad52aa

 ("media: rcar-vin: Rework parallel firmware parsing")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>