fix: Fix the issue of device unregistered message print twice

```
Device 'x' unregistered
Device 'x' unregistered
```

Signed-off-by: Jianhui Zhao <zhaojh329@gmail.com>

broker.go

@@ -133,12 +133,16 @@ func (br *broker) run() {
 			c.Close()
 
 			if c.IsDevice() {
-				if !c.(*device).registered {
+				dev := c.(*device)
+
+				if !dev.registered {
 					break
 				}
 
 				delete(br.devices, devid)
 
+				dev.registered = false
+
 				for sid, s := range br.sessions {
 					if s.dev == c {
 						s.user.Close()