igb(4): Fix VLAN support on VFs

Virtual Functions are considered untrusted and have no control
over VLAN filtering configuration in HW. To allow using
VLANs on VF intreface driver has to assume that VLAN HW Filtering
is always enabled and pass requests for adding or removing VLAN
tags to Physical Function driver using Mailbox API.

Signed-off-by: Krzysztof Galazka <krzysztof.galazka@intel.com>

Approved by: kbowling (mentor)
Reviewed by: erj (previous version)
Tested by: gowtham.kumar.ks_intel.com
MFC after: 1 week
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D53245