fix (tongou_sa1_wifi_energy_meter): charge energy depends on prepay

Make the Charge energy number entity available only when meter
is set to prepay.

custom_components/tuya_local/devices/tongou_sa1_wifi_energy_meter.yaml

@@ -136,3 +136,10 @@ entities:
         range:
           min: 0
           max: 999999
+      - id: 11
+        type: boolean
+        optional: true
+        name: available
+        mapping:
+          - dps_val: null
+            value: false