RDMA/hns: Simplify the calculation and usage of wqe idx for post verbs

RDMA/hns: Simplify the calculation and usage of wqe idx for post verbs

Currently, the wqe idx is calculated repeatly everywhere it is used.  This
patch defines wqe_idx and calculated it only once, then just use it as
needed.

Fixes: 2d40788825ac ("RDMA/hns: Add support for processing send wr and receive wr")
Link: https://lore.kernel.org/r/1575981902-5274-1-git-send-email-liweihang@hisilicon.com
Signed-off-by: Yixian Liu <liuyixian@huawei.com>
Signed-off-by: Weihang Li <liweihang@hisilicon.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>