igb: fix static function warnings reported by sparse

igb_update/validate_nvm_checksum_with_offset() should be static.
Also removes unneeded prototypes for the above functions.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>