net: dsa: return after mdb prepare phase

The current code does not return after successfully preparing the MDB
addition on every ports member of a multicast group. Fix this.

Fixes: a1a6b7ea ("net: dsa: add cross-chip multicast support")
Reported-by: Egil Hjelmeland <privat@egil-hjelmeland.no>
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/switch.c

@@ -133,6 +133,8 @@ static int dsa_switch_mdb_add(struct dsa_switch *ds,
 			if (err)
 				return err;
 		}
+
+		return 0;
 	}
 
 	for_each_set_bit(port, group, ds->num_ports)