net: dsa: return success if there was nothing to do

Clang static analysis reports this representative issue
dsa.c:486:2: warning: Undefined or garbage value
  returned to caller
  return err;
  ^~~~~~~~~~

err is only set in the loop.  If the loop is empty,
garbage will be returned.  So initialize err to 0
to handle this noop case.
Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/dsa.c

@@ -471,7 +471,7 @@ int dsa_port_walk_fdbs(struct dsa_switch *ds, int port, dsa_fdb_walk_cb_t cb)
 {
 	struct dsa_port *dp = dsa_to_port(ds, port);
 	struct dsa_mac_addr *a;
-	int err;
+	int err = 0;
 
 	mutex_lock(&dp->addr_lists_lock);
 
@@ -491,7 +491,7 @@ int dsa_port_walk_mdbs(struct dsa_switch *ds, int port, dsa_fdb_walk_cb_t cb)
 {
 	struct dsa_port *dp = dsa_to_port(ds, port);
 	struct dsa_mac_addr *a;
-	int err;
+	int err = 0;
 
 	mutex_lock(&dp->addr_lists_lock);