RDMA/hns: Refator workqueue for flush cqe process

driver inclusion
category: bugfix
bugzilla: NA
CVE: NA

Workqueue is used to process flush cqe, however, there is no need
to create one workqueue for each qp and cq as in current driver.
This patch refactors related code to simply create only one
workqueue for the hr_dev.

Signed-off-by: Yixian Liu <liuyixian@huawei.com>
Reviewed-by: oulijun <oulijun@huawei.com>
Reviewed-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>