net: dsa: mv88e6xxx: unregister mv88e6352 driver

Add the missing unregister for the mv88e6352_switch_driver.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>