net: ethernet: bcmgenet: 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.

Note that we're still leaking any fixed-link PHY registered in the
non-OF probe path.

Fixes: 9abf0c2b ("net: bcmgenet: use the new fixed PHY helpers")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>