monitor-utils: make cam_api_data a local variable

Otherwise it is global and it retains a reference to the `parent`
WpSpaDevice object until this function is called again, which prevents
some camera nodes - in some cases - from being destroyed

Fixes #640

src/scripts/lib/monitor-utils.lua

@@ -161,7 +161,7 @@ function mutils.register_cam_node (self, parent, id, factory, properties)
     cam_data.dev_path = properties["api.v4l2.path"]
   end
 
-  cam_api_data = cam_data[api]
+  local cam_api_data = cam_data[api]
   cam_api_data.enum_status = true
 
   -- cache info, it comes handy when creating node