net: dsa: add missing dsa_switch mdiobus remove

To prevent memory leakage on unbinding, add missing mdiobus unregister
and unallocation calls.

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>