net: ethernet: aurora: nb8800: fix fixed-link phydev leaks

Make sure to deregister and free any fixed-link PHY registered using
of_phy_register_fixed_link() on probe errors and on driver unbind.

Fixes: c7dfe3ab ("net: ethernet: nb8800: support fixed-link DT
node")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>