net: dsa: Add helper to convert from devlink to ds

Given a devlink instance, return the dsa switch it is associated to.

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

include/net/dsa.h

@@ -664,6 +664,13 @@ struct dsa_devlink_priv {
 	struct dsa_switch *ds;
 };
 
+static inline struct dsa_switch *dsa_devlink_to_ds(struct devlink *dl)
+{
+	struct dsa_devlink_priv *dl_priv = devlink_priv(dl);
+
+	return dl_priv->ds;
+}
+
 struct dsa_switch_driver {
 	struct list_head	list;
 	const struct dsa_switch_ops *ops;