net/mlx4_core: fix a memory leak bug.

In function mlx4_opreq_action(), pointer "mailbox" is not released,
when mlx4_cmd_box() return and error, causing a memory leak bug.
Fix this issue by going to "out" label, mlx4_free_cmd_mailbox() can
free this pointer.

Fixes: fe6f700d

 ("net/mlx4_core: Respond to operation request by firmware")
Signed-off-by: Qiushi Wu <wu000273@umn.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>