net: dsa: slave: fix fixed-link phydev leaks

Make sure to deregister and free any fixed-link PHY registered using
of_phy_register_fixed_link() on slave-setup errors and on slave destroy.

Fixes: 0d8bcdd3 ("net: dsa: allow for more complex PHY setups")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>