RDMA/hns: Fix RoCEv1 failure due to DSCP

RDMA/hns: Fix RoCEv1 failure due to DSCP

DSCP is not supported in RoCEv1, but get_dscp() is still called. If
get_dscp() returns an error, it'll eventually cause create_ah to fail
even when using RoCEv1.

Correct the return value and avoid calling get_dscp() when using
RoCEv1.

Fixes: ee20cc17e9d8 ("RDMA/hns: Support DSCP")
Signed-off-by: Junxian Huang <huangjunxian6@hisilicon.com>
Link: https://patch.msgid.link/20260104064057.1582216-4-huangjunxian6@hisilicon.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>