cc3200: Make socket.listen([backlog]) compliant with Python 3.5.

cc3200/mods/modusocket.c

@@ -185,17 +185,23 @@ STATIC mp_obj_t socket_bind(mp_obj_t self_in, mp_obj_t addr_in) {
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_2(socket_bind_obj, socket_bind);
 
-// method socket.listen(backlog)
-STATIC mp_obj_t socket_listen(mp_obj_t self_in, mp_obj_t backlog) {
-    mod_network_socket_obj_t *self = self_in;
+// method socket.listen([backlog])
+STATIC mp_obj_t socket_listen(mp_uint_t n_args, const mp_obj_t *args) {
+    mod_network_socket_obj_t *self = args[0];
+
+    int32_t backlog = 0;
+    if (n_args > 1) {
+        backlog = mp_obj_get_int(args[1]);
+        backlog = (backlog < 0) ? 0 : backlog;
+    }
 
     int _errno;
-    if (wlan_socket_listen(self, mp_obj_get_int(backlog), &_errno) != 0) {
+    if (wlan_socket_listen(self, backlog, &_errno) != 0) {
         nlr_raise(mp_obj_new_exception_arg1(&mp_type_OSError, MP_OBJ_NEW_SMALL_INT(-_errno)));
     }
     return mp_const_none;
 }
-STATIC MP_DEFINE_CONST_FUN_OBJ_2(socket_listen_obj, socket_listen);
+STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(socket_listen_obj, 1, 2, socket_listen);
 
 // method socket.accept()
 STATIC mp_obj_t socket_accept(mp_obj_t self_in) {