igb: Cleanup protocol handling in transmit path

This change is meant to cleanup the protocol handling in the transmit path
so that it correctly offloads software VLAN tagged frames.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>