[media] v4l: vsp1: video: Fix coding style

Commit 54b5a749 ("[media] v4l: vsp1: Use media entity enumeration
interface") wasn't aligned with the driver coding style. Fix it by
renaming the rval variable to ret.

Furthermore shorten lines by accessing the media_device instance in a
more straightforward fashion.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>