phy: phy-core: use the np present in of_phandle_args to get the PHY

Instead of using the node pointer of the PHY provider and then scanning its
child nodes to get a reference to the PHY, directly use the node pointer
present in of_phandle_args to get a reference to the PHY.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>