fix(vonroc_wifi_heater): use lowercase for AF preset

Original submission did not come with details, but issue #6458 provided the
Query Things Data Model snippet showing the range of values for this field
for CH510 which also identifies with the same product id.

While it is possible that earlier firmware did use uppercase AF, since there was
no evidence provided it is preferable to treat this as a fix rather than fork
the config for duplicate product ids with this minor difference in spec.

Issue #6458

custom_components/tuya_local/devices/vonroc_wifi_heater.yaml

@@ -39,7 +39,7 @@ entities:
           - dps_val: "0"
             value: "comfort"
             available: numeric_presets
-          - dps_val: AF
+          - dps_val: af
             value: away
             available: text_presets
           - dps_val: low