net/mlx5_core: Avoid copying outbox in aysnc command completion

Avoid copying to the output buffer in cmd_exec since this is done after the
command is completed. Failure to do this may cause cases where the callback
handler is called before the copy done by cmd_exec which then overwrites it.

Reported-by: Tamer Hleihel <tamerh@mellanox.com>
Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>