net: hns: remove redundant hns_mac_dev_to_enet_if()

The sequence of hns_mac_dev_to_enet_if() is the same as
hns_get_enet_interface(), and hns_get_enet_interface() is called
by initialization to get the mac mode. And the mode is not changed
anywhere. Thus add hns_mac_dev_to_enet_if() function to get the mac
mode is obviously redundant.

Reported-by: Jinchuan Tian <tianjinchuan1@huawei.com>
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>