[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>

drivers/media/dvb-core/dvb_frontend.h

@@ -643,6 +643,7 @@ struct dtv_frontend_properties {
 /**
  * struct dvb_frontend - Frontend structure to be used on drivers.
  *
+ * @refcount:		pointer to struct kref
  * @ops:		embedded struct dvb_frontend_ops
  * @dvb:		pointer to struct dvb_adapter
  * @demodulator_priv:	demod private data