// form reference
@ViewChild('form') form;
// reference to update observable
update$: Observable<any> ;

ngOnInit() {
    Observable.fromEvent(this.form.nativeElement, 'submit')
        .do(() => {
            // get values from form
        })
        .skipUntil(this.update$)
        .mergeMap(() => {
            this.update$ = this.datasource.update(..);
            return this.update$;
        })
        .subscribe()
}

You provided 'undefined' where a stream was expected.