mlx4: corretly check failed allocation

When allocation fails, mlx4_alloc_cmd_mailbox returns -ENOMEM.
Since there is no case that mlx4_alloc_cmd_mailbox returns NULL,
it needs to be checked by IS_ERR, not IS_ERR_OR_NULL
Signed-off-by: NInsu Yun <wuninsu@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/mr.c

@@ -318,7 +318,7 @@ int mlx4_mr_hw_get_mpt(struct mlx4_dev *dev, struct mlx4_mr *mmr,
 				key, NULL);
 	} else {
 		mailbox = mlx4_alloc_cmd_mailbox(dev);
-		if (IS_ERR_OR_NULL(mailbox))
+		if (IS_ERR(mailbox))
 			return PTR_ERR(mailbox);
 
 		err = mlx4_cmd_box(dev, 0, mailbox->dma, key,