Achieve hr_reg_write() to simply the codes to fill fields.

Link: https://lore.kernel.org/r/1611395282-991-4-git-send-email-liweihang@huawei.comSigned-off-by: Lang Cheng <chenglang@huawei.com>
Signed-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>