net: hns: enet specify a reference to dsaf by fwnode_handle

As device_node is only used by DT case, it is expected to find uniform
ways. So fwnode_handle is the suitable method.

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>