RDMA/hns: Refactor cmd init and mode selection for hip08

driver inclusion
category: bugfix
bugzilla: NA
CVE: NA

This patch refactor the initialization of cmd, and also for the cmd
mode selection on event and poll mode.

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>