igb: VFTA Table Fix for i350 devices

Due to a hardware problem, writes to the VFTA register can
theoretically fail. Although the likelihood of this is very low.
This patch adds a shadow vfta in the adapter struct for reading
and adds new write functions for these devices to work around the problem.

Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>