dsa: Destroy fixed link phys after the phy has been disconnected

The phy is disconnected from the slave in dsa_slave_destroy(). Don't
destroy fixed link phys until after this, since there can be fixed
linked phys connected to ports.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>