net/mlx5: Fix error flow in CREATE_QP command

In error flow, when DESTROY_QP command should be executed, the wrong
mailbox was set with data, not the one that is written to hardware,
Fix that.

Fixes: 09a7d9ec '{net,IB}/mlx5: QP/XRCD commands via mlx5 ifc'
Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>