SmartMi E1: move `class: duration` to correct location

Was placed as a state class, instead of the intended device class.

custom_components/tuya_local/devices/smartmi_e1_purifier.yaml

@@ -34,10 +34,10 @@ secondary_entities:
   - entity: sensor
     name: Filter usage
     category: diagnostic
+    class: duration
     icon: "mdi:shield-bug"
     dps:
       - id: 101
         name: sensor
         type: integer
         unit: h
-        class: duration