net: ethernet: mtk_eth_soc: handle VLAN pop action

Do not hit EOPNOTSUPP when flowtable offload provides a VLAN pop action.

Fixes: efce49df ("netfilter: flowtable: add vlan pop action offload support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>