Fix ZHA force polled entities. (#22222)

## Description:
Fix "force_polled" ZHA entities.

## Checklist:
  - [x] The code change is tested and works locally.
  - [x] Local tests pass with `tox`. **Your PR cannot be merged unless tests pass**
  - [x] There is no commented out code in this PR.

[ex-requir]: https://github.com/home-assistant/home-assistant/blob/dev/homeassistant/components/keyboard/__init__.py#L14
[ex-import]: https://github.com/home-assistant/home-assistant/blob/dev/homeassistant/components/keyboard/__init__.py#L23

homeassistant/components/zha/entity.py

@@ -179,7 +179,7 @@ def async_restore_last_state(self, last_state):
 
     async def async_update(self):
         """Retrieve latest state."""
-        for channel in self.cluster_channels:
+        for channel in self.cluster_channels.values():
             if hasattr(channel, 'async_update'):
                 await channel.async_update()