ixgbe: fix mdio bus registration

The ixgbe ignores errors returned from mdiobus_register() and leaves
adapter->mii_bus non-NULL and MDIO bus state as MDIOBUS_ALLOCATED.
This triggers a BUG from mdiobus_unregister() during ixgbe_remove() call.

Fixes: 8fa10ef0 ("ixgbe: register a mdiobus")
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/ixgbe/ixgbe_phy.c

@@ -905,13 +905,12 @@ s32 ixgbe_mii_bus_init(struct ixgbe_hw *hw)
 	struct pci_dev *pdev = adapter->pdev;
 	struct device *dev = &adapter->netdev->dev;
 	struct mii_bus *bus;
+	int err = -ENODEV;
 
-	adapter->mii_bus = devm_mdiobus_alloc(dev);
-	if (!adapter->mii_bus)
+	bus = devm_mdiobus_alloc(dev);
+	if (!bus)
 		return -ENOMEM;
 
-	bus = adapter->mii_bus;
-
 	switch (hw->device_id) {
 	/* C3000 SoCs */
 	case IXGBE_DEV_ID_X550EM_A_KR:
@@ -949,12 +948,15 @@ s32 ixgbe_mii_bus_init(struct ixgbe_hw *hw)
 	 */
 	hw->phy.mdio.mode_support = MDIO_SUPPORTS_C45 | MDIO_SUPPORTS_C22;
 
-	return mdiobus_register(bus);
+	err = mdiobus_register(bus);
+	if (!err) {
+		adapter->mii_bus = bus;
+		return 0;
+	}
 
 ixgbe_no_mii_bus:
 	devm_mdiobus_free(dev, bus);
-	adapter->mii_bus = NULL;
-	return -ENODEV;
+	return err;
 }
 
 /**