[media] v4l2 core: return -ENOTTY if an ioctl doesn't exist

Currently, -EINVAL is used to return either when an IOCTL is not
implemented, or if the ioctl was not implemented.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>