net: hns3: add support for external loopback test

This patch add support for external loopback test.
The successful test need the link is up with duplex full. The
driver do external loopback first, and then the whole offline
test.

Signed-off-by: Yonglong Liu <liuyonglong@huawei.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>