RDMA/hns: Refactor hns_roce_create_srq()

Split the SRQ creation process into multiple steps and encapsulate them
into functions.

Link: https://lore.kernel.org/r/1611997090-48820-7-git-send-email-liweihang@huawei.com


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