On igb/em interfaces we did not receive packets with VLAN tag 0 unless
vlanhwfilter was disabled.
This can be fixed by explicitly listing VLAN 0 in the hardware VLAN
filter (VFTA). Do this from em_setup_vlan_hw_support(), where we already
(re-)write the VFTA.
Sponsored by: Rubicon Communications, LLC ("Netgate")