RDMA/hns: Initialize all fields of doorbells to zero

Prevent uninitialized fields when new fields are added, and make code look
simpler.

Link: https://lore.kernel.org/r/1582162471-50361-1-git-send-email-liweihang@huawei.com


Signed-off-by: Lang Cheng <chenglang@huawei.com>
Signed-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>