fix(krain_krx8_irrigation_controller): fix signal strength sensor

As per comment on #6393

custom_components/tuya_local/devices/krain_krx8_irrigation_controller.yaml

@@ -357,7 +357,8 @@ entities:
     dps:
       - id: 134
         type: base64
-        mask: "FF00000000000000000000000000000000000000"
+        mask: "FF0000000000000000000000000000000000000000"
+        mask_signed: true
         name: sensor
         unit: dBm
         class: measurement