Staging: sxg: Add Checksum Offload support for Sahara SXG driver

* This patch adds support for offloading checksum to hardware.
  IP checksum have been tested for IPv4 and IPv6.

Signed-off-by: LinSysSoft Sahara Team <saharaproj@linsyssoft.com>
Signed-off-by: Mithlesh Thukral <mithlesh@linsyssoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>