of: mdio: Refactor of_get_phy_id()

With the introduction of fwnode_get_phy_id(), refactor of_get_phy_id()
to use fwnode equivalent.

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>