Fix bbr start

Author: Mika Tervonen

source/6LoWPAN/ws/ws_bbr_api.c

@@ -363,7 +363,7 @@ int ws_bbr_start(int8_t interface_id, int8_t bb_interface_id)
 #ifdef HAVE_WS
 
     (void)interface_id;
-    protocol_interface_info_entry_t *bb_interface = protocol_stack_interface_info_get_by_id(backbone_interface_id);
+    protocol_interface_info_entry_t *bb_interface = protocol_stack_interface_info_get_by_id(bb_interface_id);
 
     if (!bb_interface) {
         return -1;