mlx4_en: Assign dummy event handler for TX queue

The low level driver always assumes this handler exists.
The lack of it could cause kernel panic

Signed-off-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/mlx4/en_tx.c

@@ -112,6 +112,7 @@ int mlx4_en_create_tx_ring(struct mlx4_en_priv *priv,
 		mlx4_err(mdev, "Failed allocating qp %d\n", ring->qpn);
 		goto err_reserve;
 	}
+	ring->qp.event = mlx4_en_sqp_event;
 
 	return 0;