Bluetooth: controller: Stop Sync ISO ticker when establishment fails

When an ISO stream fails to sync to a broadcaster within the first 6
events, the establishment fails (as expected). However, it did not stop
the ticker, and subsequently it was impossible to establish a new sync
after this.

Make asynchronous call to ticker_stop when establishment fails, and exit
done handling. Cleanup is handled via ll_rx_dequeue().

Signed-off-by: Morten Priess <mtpr@oticon.com>