igb: use mac loopback for i354 backplane

We can't know what PHY is to be used for i354 backplane, so use MAC
loopback for ethtool tests.

Signed-off-by: Todd Fujinaka <todd.fujinaka@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>