igb: Don't let ethtool try to write to iNVM in i210/i211

Don't let ethtool try to write to iNVM in i210/i211.

This fixes an issue seen by Marek Vasut.

Reported-by: Marek Vasut <marex@denx.de>
Signed-off-by: Todd Fujinaka <todd.fujinaka@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>