net: mdio-mux: bcm-iproc: call mdiobus_free() in error path

If an error is encountered in mdio_mux_init(), the error path will call
mdiobus_free().  Since mdiobus_register() has been called prior to
mdio_mux_init(), the bus->state will not be MDIOBUS_UNREGISTERED.  This
causes a BUG_ON() in mdiobus_free().  To correct this issue, add an
error path for mdio_mux_init() which calls mdiobus_unregister() prior to
mdiobus_free().

Signed-off-by: Jon Mason <jon.mason@broadcom.com>
Fixes: 98bc865a ("net: mdio-mux: Add MDIO mux driver for iProc SoCs")
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>