This fixes: 80264a8 driver: Call bus->remove instead of driver->remove

On mvebu it happens that:

Upon device shutdown, when iterating through the active device list,
the phy0 device is removed before mdio-mvebu. Then, when the mdio bus
device is removed, the phy0 device is removed again, here:

mdio_bus_remove(on mdio-mvebu)
  mvebu_mdio_remove
    mdiobus_unregister
      unregister_device
        mdio_bus_remove(on phy0)

Fix this by setting the mdio busses phy_map[phy->addr] to NULL when
unregistering the phy device, so that mdiobus_unregister no longer
finds a valid phy_device when iterating over the busses device list.

Reported-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>