net: dsa: complete dsa_switch_destroy

When unbinding dsa, complete the dsa_switch_destroy to unregister the
fixed link phy then cleanly unregister and destroy the net devices.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>