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: Insu Yun <wuninsu@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>