drivers: net: zynq_gem: fix phy dt node setting

Now zynq_gem 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>
Tested-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>