net-next: dsa: make the set_addr() operation optional

Only 1 of the 3 drivers currently has a set_addr() operation. Make the
set_addr() callback optional to reduce the amount of empty stubs inside
the drivers.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/dsa/dsa2.c b/net/dsa/dsa2.c
index cffc19e..f8a7d9a 100644
--- a/net/dsa/dsa2.c
+++ b/net/dsa/dsa2.c
@@ -304,9 +304,11 @@
 	if (err < 0)
 		return err;
 
-	err = ds->ops->set_addr(ds, dst->master_netdev->dev_addr);
-	if (err < 0)
-		return err;
+	if (ds->ops->set_addr) {
+		err = ds->ops->set_addr(ds, dst->master_netdev->dev_addr);
+		if (err < 0)
+			return err;
+	}
 
 	if (!ds->slave_mii_bus && ds->ops->phy_read) {
 		ds->slave_mii_bus = devm_mdiobus_alloc(ds->dev);