Fix ebtables dhcpv6 rules

the ports were interchanged, see the following packet flow:

    client:546 --> [ff02::1:2]:547 -- dhcpv6 information request
    server:547 --> client:546      -- dhcpv6 reply

therefore we need to allow outgoing multicast packets with dst-port 547
and unicast packets from bat0 to clients with dst-port 546 and 547 in the other direction