net/mlx4_core: Fix memory access error in mlx4_QUERY_DEV_CAP_wrapper()

Fix a regression introduced by [1]. outbox was accessed instead of
outbox->buf. Typo was copy-pasted to [2] and [3].

[1] - cc1ade94 mlx4_core: Disable memory windows for virtual functions
[2] - 4de65803 mlx4_core: Add support for steerable IB UD QPs
[3] - 7ffdf726

 net/mlx4_core: Add basic support for TCP/IP offloads under
      tunneling

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>