of: mdio: fixup of_phy_register_fixed_link parsing of new bindings

Fixes commit 3be2a49e ("of: provide a binding for fixed link PHYs")

Fix the parsing of the new fixed link dts bindings for duplex,
pause, and asym_pause by using the correct device node pointer.

Signed-off-by: Richard Retanubun <rretanubun.work@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>