kw2xrf: Don't propagate NETOPT_CHANNEL setter

This modification changes the netdev::set function for the kw2xrf to not
propagate the NETOPT_CHANNEL to the generic netdev_ieee802154 code

drivers/kw2xrf/kw2xrf_netdev.c

@@ -427,8 +427,7 @@ static int _set(netdev_t *netdev, netopt_t opt, const void *value, size_t len)
                     res = -EINVAL;
                     break;
                 }
-                dev->netdev.chan = chan;
-                /* don't set res to set netdev_ieee802154_t::chan */
+                res = sizeof(uint16_t);
             }
             break;