RDMA/hns: Adjust fields and variables about CMDQ tail/head

The register 0x07014 is actually the head pointer of CMDQ, and 0x07010
means tail pointer. Current definitions are confusing, so rename them and
related variables.

The next_to_use of structure hns_roce_v2_cmq_ring has the same semantics
as head, merge them into one member. The next_to_clean of structure
hns_roce_v2_cmq_ring has the same semantics as tail. After deleting
next_to_clean, tail should also be deleted.

Link: https://lore.kernel.org/r/1612688143-28226-5-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@nvidia.com>