ixgbe: fix probing of multi-port devices with one MDIO

Ian reports that after upgrade from v5.8.14 to v5.9 only one
of his 4 ixgbe netdevs appear in the system.

Quoting the comment on ixgbe_x550em_a_has_mii():
 * Returns true if hw points to lowest numbered PCI B:D.F x550_em_a device in
 * the SoC.  There are up to 4 MACs sharing a single MDIO bus on the x550em_a,
 * but we only want to register one MDIO bus.

This matches the symptoms, since the return value from
ixgbe_mii_bus_init() is no longer ignored we need to handle
the higher ports of x550em without an error.

Fixes: 09ef193f ("net: ethernet: ixgbe: check the return value of ixgbe_mii_bus_init()")
Reported-by: Ian Kumlien <ian.kumlien@gmail.com>
Tested-by: Ian Kumlien <ian.kumlien@gmail.com>
Acked-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Link: https://lore.kernel.org/r/20201016232006.3352947-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -901,15 +901,13 @@ static bool ixgbe_x550em_a_has_mii(struct ixgbe_hw *hw)
  **/
 s32 ixgbe_mii_bus_init(struct ixgbe_hw *hw)
 {
+	s32 (*write)(struct mii_bus *bus, int addr, int regnum, u16 val);
+	s32 (*read)(struct mii_bus *bus, int addr, int regnum);
 	struct ixgbe_adapter *adapter = hw->back;
 	struct pci_dev *pdev = adapter->pdev;
 	struct device *dev = &adapter->netdev->dev;
 	struct mii_bus *bus;
 
-	bus = devm_mdiobus_alloc(dev);
-	if (!bus)
-		return -ENOMEM;
-
 	switch (hw->device_id) {
 	/* C3000 SoCs */
 	case IXGBE_DEV_ID_X550EM_A_KR:
@@ -922,16 +920,23 @@ s32 ixgbe_mii_bus_init(struct ixgbe_hw *hw)
 	case IXGBE_DEV_ID_X550EM_A_1G_T:
 	case IXGBE_DEV_ID_X550EM_A_1G_T_L:
 		if (!ixgbe_x550em_a_has_mii(hw))
-			return -ENODEV;
-		bus->read = &ixgbe_x550em_a_mii_bus_read;
-		bus->write = &ixgbe_x550em_a_mii_bus_write;
+			return 0;
+		read = &ixgbe_x550em_a_mii_bus_read;
+		write = &ixgbe_x550em_a_mii_bus_write;
 		break;
 	default:
-		bus->read = &ixgbe_mii_bus_read;
-		bus->write = &ixgbe_mii_bus_write;
+		read = &ixgbe_mii_bus_read;
+		write = &ixgbe_mii_bus_write;
 		break;
 	}
 
+	bus = devm_mdiobus_alloc(dev);
+	if (!bus)
+		return -ENOMEM;
+
+	bus->read = read;
+	bus->write = write;
+
 	/* Use the position of the device in the PCI hierarchy as the id */
 	snprintf(bus->id, MII_BUS_ID_SIZE, "%s-mdio-%s", ixgbe_driver_name,
 		 pci_name(pdev));