net/mlx5: Report a CQ error event only when a handler was set

Report a CQ error event only when a handler was set.

This enables mlx5_ib to not set a handler upon CQ creation and use some
other mechanism to get this event as of other events by the
mlx5_eq_notifier_register API.

Signed-off-by: Yishai Hadas <[e-mail]>
Acked-by: Saeed Mahameed <[e-mail]>
Signed-off-by: Leon Romanovsky <[e-mail]>

drivers/net/ethernet/mellanox/mlx5/core/eq.c

···
 		return NOTIFY_OK;
 	}
 
-	cq->event(cq, type);
+	if (cq->event)
+		cq->event(cq, type);
 
 	mlx5_cq_put(cq);