DualR3 not syncing with homebridge
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### What issue do you have? Please be as thorough and explicit as possible.
Hi. I set up a Sonoff DualR3 on a curtain motor. Linked and calibrated with the ewelink app no issues. 
After configuring on homebridge though, I can control the curtain in HomeKit, but the positions do not seem to update.
If I close the curtain, HomeKit keeps hanging on "closing" as the screenshot below. If I restart homebridge server, then the position is correctly updated.

<img width="114" alt="Screenshot 2021-07-03 at 00 03 44" src="https://user-images.githubusercontent.com/86857167/124332685-51dbce80-db92-11eb-9796-c4fc4b37a79d.png">



### Details of your setup.
* Do you use (1) Homebridge (+ config-ui), (2) Homebridge (CLI) or (3) HOOBS? 

Homebridge + config-ui


* Which version of Homebridge/HOOBS do you have?

1.3.4


* Which platform do you run Homebridge/HOOBS on (e.g. Raspberry Pi/Windows/HOOBS Box)? Please also mention your version of Node.js/NPM if known.

Raspberry Pi
Node.js Version v14.17.1
Npm Version v7.17.0



* Which version of this plugin (homebridge-ewelink) do you have? Has the issue started since upgrading from a previous version?

6.8.0
Only tried on this version

* Which eWeLink devices do you have that are causing issues? Please include product models if applicable.

Sonoff DualR3


### Please paste any relevant logs below.
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```
"multiDevices": [
                {
                    "deviceId": "xxxxxxxxxxx",
                    "label": "Bedroom Shutter"
                }
            ],
```
