RDMA/hns: Refactor process about opcode in post_send()

According to the IB specifications, the verbs should return an immediate
error when the users set an unsupported opcode. Furthermore, refactor codes
about opcode in process of post_send to make the difference between opcodes
clearer.

Link: https://lore.kernel.org/r/1600509802-44382-2-git-send-email-liweihang@huawei.com
Signed-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>