When netdev is down, the stack will delete the vlan from
hardware including vlan0, which will cause problem when
doing loopback selftest when netdev is down.

This patch fixes it by always preserving vlan 0 in hardware,
because vlan 0 is defalut vlan, which should always be in
hardware.

Fixes: c39c4d98 ("net: hns3: Add mac loopback selftest support in hns3 driver")
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>