net: phy: fixed_phy: fix of_node leak in fixed_phy_unregister

Make sure to drop the of_node reference taken in fixed_phy_register()
when deregistering a PHY.

Fixes: a7595121 ("net: phy: extend fixed driver with
fixed_phy_register()")

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>