RDMA/hns: Refactor hns_roce_v2_set_hem()

The parts about preparing and sending mailbox to hardware is not strongly
related to other codes in hns_roce_v2_set_hem(), and can be encapsulated
into a separate function.

Link: https://lore.kernel.org/r/1595932941-40613-3-git-send-email-liweihang@huawei.com


Signed-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>