igc: Add support for TSO

TCP segmentation offload allows a device to segment a single frame
into multiple frames with a data payload size specified in socket buffer.
As a result we can now send data approximately up to seven percents fast
than was previously possible on my system.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>