net: hisilicon: cleanup to prepare for other cases

Hns-mdio only supports DT case now. do some cleanup to prepare
for introducing other cases later, no functional change.

Signed-off-by: Kejian Yan <yankejian@huawei.com>
Signed-off-by: Yisen Zhuang <Yisen.Zhuang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>