e1000: fix VLAN 0

VLAN 0 essentially means "Treat as untagged, but with priority bits",
and is used by some ISPs.

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.

Reviewed by: kbowling
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D40046