IB/mlx5: Refactor CQE compression response

Refactor CQE compression response to be fully set only
when it`s really supported. There is no change from user
perspective because anyway resp.cqe_comp_caps.max_num was
set to zero.

Reviewed-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Yonatan Cohen <yonatanc@mellanox.com&gt;W>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>