igc: Add Rx checksum support

Extend the socket buffer field process and add Rx checksum functionality
Minor: fix indentation with tab instead of spaces.

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>