igb: Retain HW VLAN filtering while in promiscuous + VT mode

When using the new bridge FDB interface to allow SR-IOV virtual function
network devices to communicate with SW bridged network devices the
physical function is placed into promiscuous mode and hardware VLAN
filtering is disabled.  This defeats the ability to use VLAN tagging
to isolate user networks.  When the device is in promiscuous mode and
VT mode simultaneously ensure that VLAN hardware filtering remains
enabled.

Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>