RDMA/mlx5: Fix typo in destroy_mkey inbox

Set mkey_index to the correct place when preparing the destroy_mkey inbox,
this will fix the following syndrome:

 mlx5_core 0000:08:00.0: mlx5_cmd_check:782:(pid 980716): DEALLOC_PD(0x801)
 op_mod(0x0) failed, status bad resource state(0x9), syndrome (0x31b635)

Link: https://lore.kernel.org/r/20210311142024.1282004-1-leon@kernel.org
Fixes: 1368ead0

 ("RDMA/mlx5: Use strict get/set operations for obj_id")
Reviewed-by: Ido Kalir <idok@mellanox.com>
Reviewed-by: Yishai Hadas <yishaih@nvidia.com>
Signed-off-by: Mark Zhang <markzhang@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>