igb: Fix ethtool loopback test for 82580 copper

Add back 82580 loopback tests to ethtool.
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>

drivers/net/ethernet/intel/igb/igb_ethtool.c

@@ -1607,6 +1607,9 @@ static int igb_integrated_phy_loopback(struct igb_adapter *adapter)
 			igb_write_phy_reg(hw, I347AT4_PAGE_SELECT, 0);
 			igb_write_phy_reg(hw, PHY_CONTROL, 0x4140);
 		}
+	} else if (hw->phy.type == e1000_phy_82580) {
+		/* enable MII loopback */
+		igb_write_phy_reg(hw, I82580_PHY_LBK_CTRL, 0x8041);
 	}
 
 	/* add small delay to avoid loopback test failure */