net: of/phy: fix build error when phylib is built as a module

CONFIG_OF_MDIO is tristate and will be m if PHYLIB is m.  Use
IS_ENABLED macro to prevent build error:

 ERROR: "of_mdio_find_bus" [drivers/net/phy/mdio-mux.ko] undefined!

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Cc: David Daney <david.daney@cavium.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Acked-by: David Daney <david.daney@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>