net: hns: add attribute port-mode-offset for dsaf port node

Port mode offset for each dsaf port is different. The current code is not
so readability. This patch adds configuration named port-mode-offset to
make the code simple and more readability. If port-mode-offset isn't
exists, default value 0 will be used.

Signed-off-by: Daode Huang <huangdaode@hisilicon.com>
Signed-off-by: Yisen Zhuang <Yisen.Zhuang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>