net/mlx5_core: Fix async commands return code

In case of async command completion, the error code returned should take
into account the command completion status.

Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>