RDMA/hns: Update posting & querying mailbox

This patch updates the implementation of the mailbox command interface by
using command queue instead of operating registers.  With this update, the
software can be well decoupled with the hardware.

Signed-off-by: Lijun Ou <oulijun@huawei.com>
Signed-off-by: Yixian Liu <liuyixian@huawei.com>
Signed-off-by: Wei Hu (Xavier) <xavier.huwei@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>