igc: Clean up obsolete NVM defines

Packet buffer allocation, reserved word and pointer guard
not applicable for i225 parts.
This patch comes to clean up these obsolete defines

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>