phy: mdio-mux: fix Kconfig dependencies

MDIO_BUS_MUX can only be selected if OF_MDIO is already turned on:

WARNING: unmet direct dependencies detected for MDIO_BUS_MUX
  Depends on [n]: NETDEVICES [=y] && MDIO_BUS [=m] && OF_MDIO [=n]
  Selected by [m]:
  - MDIO_BUS_MUX_MULTIPLEXER [=m] && NETDEVICES [=y] && MDIO_BUS [=m] && OF [=y]

Fixes: 7865ad65 ("drivers: net: phy: mdio-mux: Add support for Generic Mux controls")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>