RDMA/hns: Do shift on traffic class when using RoCEv2

The high 6 bits of traffic class in GRH is DSCP (Differentiated Services
Codepoint), the driver should shift it before the hardware gets it when
using RoCEv2.

Fixes: 606bf89e ("RDMA/hns: Refactor for hns_roce_v2_modify_qp function")
Fixes: fba429fc

 ("RDMA/hns: Fix missing fields in address vector")
Link: https://lore.kernel.org/r/1607650657-35992-4-git-send-email-liweihang@huawei.com
Signed-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>