mgb: enable multicast in mgb_init

Receive Filtering Engine (RFE) configuration is not yet implemented,
and mgb intended to enable all broadcast, multicast, and unicast.
However, MGB_RFE_ALLOW_MULTICAST was missed (MGB_RFE_ALLOW_UNICAST was
included twice).

MFC after: 1 week
Fixes: 8890ab7758b8 ("Introduce if_mgb driver...")
Sponsored by: The FreeBSD Foundation