net: ethernet: mediatek: fix fixed-link phydev leaks

Make sure to deregister and free any fixed-link PHY registered using
of_phy_register_fixed_link() on initialisation errors and on uninit.

Fixes: 0c72c50f ("net-next: mediatek: add fixed-phy support")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>