net: dsa: Add teardown callback for drivers

This is helpful for e.g. draining per-driver (not per-port) tagger
queues.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/dsa.h

@@ -357,6 +357,7 @@ struct dsa_switch_ops {
 						  int port);
 
 	int	(*setup)(struct dsa_switch *ds);
+	void	(*teardown)(struct dsa_switch *ds);
 	u32	(*get_phy_flags)(struct dsa_switch *ds, int port);
 
 	/*