net: hns3: Add mac loopback selftest support in hns3 driver

This patch adds mac loopback selftest support for ethtool cmd
by checking if a transmitted packet can be received correctly
when mac loopback is enabled.

Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>