[media] dvb_frontend: add kernel-doc tag for a missing parameter

Changeset 1f862a68 ("[media] dvb_frontend: move kref to struct
dvb_frontend") added a kref to the struct dvb_frontend, but it
didn't document it.

Fixes: 1f862a68 ("[media] dvb_frontend: move kref to struct dvb_frontend")

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>