net: dsa: microchip: add port_cleanup function

Add port_cleanup function to reset some device variables when the port is
disabled.  Add a mutex to make sure changing those variables is
thread-safe.

Signed-off-by: Tristram Ha <Tristram.Ha@microchip.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>