RDMA/hns: Modify the usage of cmd_sn in hip08

RDMA/hns: Modify the usage of cmd_sn in hip08

The cmd_sn field of CQ doorbell inits for 0. It should be
increment on each first db rung after a completion Event.
if the cmd_sn of notify doorbell Adjacent two times is the
same, the hardware will distinguish it for the same notify
request and update its type according to the priority level
of next event and solicited event.

Signed-off-by: Lijun Ou <oulijun@huawei.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>