[media] v4l2-device: fix a missing error code

We need to set "err = -ENOMEM" here.

Fixes: 38b11f19667a ('[media] v4l2-core: create MC interfaces for devnodes')

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>