media: devnode: Rename mdev argument as devnode

Historically, mdev argument name was being used on both struct
media_device and struct media_devnode. Recently most occurrences of mdev
referring to struct media_devnode were replaced by devnode, which makes
more sense. Fix the last remaining occurrence.

Fixes: 163f1e93 ("[media] media-devnode: fix namespace mess")

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>