RDMA/hns: Simplify process of filling UD SQ WQE

There are some codes can be simplified or encapsulated in set_ud_wqe() to
make them easier to be understand.

Link: https://lore.kernel.org/r/1605526408-6936-6-git-send-email-liweihang@huawei.com
Signed-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
