[media] cec: fix ioctl return code when not registered

Don't return the confusing -EIO error code when the device is not registered,
instead return -ENODEV which is the proper thing to do in this situation.

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