igb: Update firmware version info for ethtool output.

There are multiple places in our device nvm where the version is stored.
This update fixes some output errors with some types of images and
refactors the way the version data is gathered and stored for ethtool output.

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>