igb: Replace mdelay() with msleep() in igb_integrated_phy_loopback()

igb_integrated_phy_loopback() is never called in atomic context.
It calls mdelay() to busily wait, which is not necessary.
mdelay() can be replaced with msleep().

This is found by a static analysis tool named DCNS written by myself.
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.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

@@ -1649,7 +1649,7 @@ static int igb_integrated_phy_loopback(struct igb_adapter *adapter)
 	if (hw->phy.type == e1000_phy_m88)
 		igb_phy_disable_receiver(adapter);
 
-	mdelay(500);
+	msleep(500);
 	return 0;
 }