mgr/dashboard: close token status subscription properly

Since its not returning any subscription back to the `this.subs`
property, those subscriptions are not properly closed in the
workbench-layout when its destroyed. So ensuring proper return

Fixes: https://tracker.ceph.com/issues/72530
Signed-off-by: Nizamudeen A <[email protected]>

Author: nizamial09

src/pybind/mgr/dashboard/frontend/src/app/shared/api/multi-cluster.service.ts

@@ -33,12 +33,12 @@ export class MultiClusterService {
   }
 
   startClusterTokenStatusPolling() {
-    this.checkAndStartTimer();
+    return this.checkAndStartTimer();
   }
 
   private checkAndStartTimer() {
     this.checkTokenStatus().subscribe(this.getClusterTokenStatusObserver());
-    this.timerService
+    return this.timerService
       .get(() => this.checkTokenStatus(), this.TOKEN_CHECK_INTERVAL)
       .subscribe(this.getClusterTokenStatusObserver());
   }