netdev/phy: Prototype of_mdio_find_bus()

Ensure that of_mdio_find_bus() matches the prototype in the header (and
stop sparse complaining) by including the header with the prototype.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: David S. Miller <davem@davemloft.net>