cc2538_rf: Don't propagate channel to 802154 layer

cpu/cc2538/radio/cc2538_rf_netdev.c

@@ -175,6 +175,7 @@ static int _set(netdev_t *netdev, netopt_t opt, const void *value, size_t value_
                 }
                 else {
                     cc2538_set_chan(chan);
+                    res = sizeof(uint16_t);
                 }
             }
             break;