igc: Clean up nvm_info structure

flash_bank_size and flash_base_addr field not in use and can
be removed from a nvm_info structure

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>