RDMA/hns: Simplify doorbell initialization code

If a variable needs to be set to 0 before use, it can be directly
initialized to 0.

Link: https://lore.kernel.org/r/1572952082-6681-7-git-send-email-liweihang@hisilicon.com


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