igb: Fix EEPROM writes via ethtool on i210

This patch fixes a problem where the driver would crash when trying to
write a word to the EEPROM on i210 devices.

Reported-by: Ekman Tsang <Ekman.Tsang@riverbed.com>
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>