igb: Refactor VFTA configuration

igb: Refactor VFTA configuration

BugLink: http://bugs.launchpad.net/bugs/1547674
This patch starts the clean-up process on the VFTA configuration.
Specifically in this patch I attempt to address and simplify several items
while also updating the code to bring it more inline with what is already
in ixgbe.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
(cherry picked from net-next commit 832e821c51e381966464c8a0f30f12eb1514eba0)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>