RDMA/hns: Delete unnecessary variable max_post

There is no need to define max_post in hns_roce_wq, as it does same thing
as wqe_cnt.

Link: https://lore.kernel.org/r/1572952082-6681-2-git-send-email-liweihang@hisilicon.com


Signed-off-by: Yixian Liu <liuyixian@huawei.com>
Signed-off-by: Weihang Li <liweihang@hisilicon.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>