RDMA/hns: Refactor for hns_roce_v2_modify_qp function

Move some lines which exist hns_roce_v2_modify_qp function into a new
function. The code refactored mainly includes some absolute fields of qp
context and some optional fields of qp context.

Link: https://lore.kernel.org/r/1562593285-8037-4-git-send-email-oulijun@huawei.comSigned-off-by: Lijun Ou <oulijun@huawei.com>
Signed-off-by: Xi Wang <wangxi11@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>