net: ethernet: marvell: mvneta: 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: 83895bed ("net: mvneta: add support for fixed links")
Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>