Rewrite the multicast/promiscuous handling and in the process merge into

Rewrite the multicast/promiscuous handling and in the process merge into
one function. Also fix reception of multicast packets when in promisuous
mode. The Realtek chips are a little different than most others in that
they will not receive multicast packets by enabling promiscuous mode
alone.

Tested by naddy@ and sthen@
ok naddy@