net: phy: Use devm api for mdio bus allocation in bcm iproc mdio mux

Use devm_mdiobus_alloc() instead of mdiobus_alloc() in the Broadcom
iProc mdio mux driver.

Signed-off-by: Arun Parameswaran <arun.parameswaran@broadcom.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>