RDMA/hns: Refactor hns_roce_v2_poll_one()

Encapsulate the process of obtaining the current QP and filling WC as
functions, also merge some duplicate code.

Link: https://lore.kernel.org/r/1616815294-13434-2-git-send-email-liweihang@huawei.comSigned-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>