igb: Add function to read i211's invm version

The i211's one-time programmable (invm) version field is different than the
other fields contained in it.  This patch adds a function to get the invm version
of it and store it for output from ethtool.

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>