igb: rework handling of the vfta and vlvf registers in relation to mng_vlan

This patch corrects some errors in how vlans are being handled when vfs
start interacting with the management vlans.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>