Heatstorm: correct type of brightness dp

Type is string, not integer.  When set to integer, it fails when trying to
ensure the value is rounded if the value is "off".

Issue #510

custom_components/tuya_local/devices/heatstorm_hs6000gc_heater.yaml

@@ -77,7 +77,7 @@ secondary_entities:
     dps:
       - id: 5
         name: brightness
-        type: integer
+        type: string
         mapping:
           - dps_val: "Off"
             value: 0