RDMA/mlx5: Use strict get/set operations for obj_id

Use strict get/set operations for obj_id based on the specific object
type. This comes to prevent any miss match between the general header to
the legacy header commands.

Link: https://lore.kernel.org/r/20201230130121.180350-4-leon@kernel.org
Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>