net: hns3: Add support for tx_accept_tag2 and tx_accept_untag2 config

HNS3 Hardware can support up to two VLAN tags in transmit leg, the PPP
module can handle the packets based on the tag1 and tag2 config. This
patch adds support for tag2 config for vlan handling

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>