RDMA/hns: Refactor hns_roce_v2_set_hem for hip08

In order to reduce the complexity of hns_roce_v2_set_hem, extract
the implementation of op as a function.

Signed-off-by: Yangyang Li <liyangyang20@huawei.com>
Link: https://lore.kernel.org/r/1565276034-97329-12-git-send-email-oulijun@huawei.com


Signed-off-by: Doug Ledford <dledford@redhat.com>