net: phy: Introduce fwnode_mdio_find_device()

Define fwnode_mdio_find_device() to get a pointer to the
mdio_device from fwnode passed to the function.

Refactor of_mdio_find_device() to use fwnode_mdio_find_device().

Signed-off-by: Calvin Johnson <calvin.johnson@oss.nxp.com>
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Acked-by: Grant Likely <grant.likely@arm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>