igb: Refactor NVM read functions to accommodate devices with no flash

This patch refactors NVM read functions in order to accommodate i210 devices
that do not have a flash.  Previously, this was not supported on i210
devices.

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>