igb: Fix set_ethtool function to call update nvm for entire image

This patch fixes a problem where we were only checking to update checksum
on first part of nvm image.  Newer parts have multiple checksum fields and
checksum function will accommodate that as long as we call it in the first
place for any changes made.

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>