igb: Add loopback test support for i210

Early release of i210 devices had the loopback test of the ethtool
self-test disabled. This patch enables the loopback test for i210 devices.

Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>