Fix ixgbe(4) support for ifconfig's vlanhwtag flag. Disabling this flag
will now prevent the driver from stripping vlan tags from packets.

PR: 219390
Submitted by: Piotr Pietruszewski <piotr.pietruszewski@intel.com>
Reported by: Charles Goncalves <halfling@halfling.com.br>
Obtained from: 1 week
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D12795