net: hns3: add support for dump MAC ID and loopback status in debugfs

The MAC ID and loopback status information are obtained from
the hardware, which will be helpful for debugging. This patch
adds support for these two items in debugfs.

Signed-off-by: Yufeng Mo <moyufeng@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>