drivers: net: cpsw: fix phy dt node setting

Now CPSW driver will overwrite UCLASS_ETH node when PHY is
connected and configured which is not correct.
Use struct phydev->node instead.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>