drv/bluetooth_btstack: finish TODO to wait for setting advertising data

This finishes up the REVISIT comment in start_broadcasting_task() to
actually wait for the set advertising data command to complete before
the task completes.

This fixes issues with calling the task too fast while the commands
from the previous call are still pending.

The timer was used to work around the issue but is no longer needed.

Fixes: pybricks/support#1151

lib/pbio/drv/bluetooth/bluetooth_btstack.c

@@ -739,8 +739,6 @@ void pbdrv_bluetooth_disconnect_remote(void) {
 static PT_THREAD(start_broadcasting_task(struct pt *pt, pbio_task_t *task)) {
     pbdrv_bluetooth_value_t *value = task->context;
 
-    static struct timer broadcast_delay;
-
     PT_BEGIN(pt);
 
     if (value->size > LE_ADVERTISING_DATA_SIZE) {
@@ -761,14 +759,9 @@ static PT_THREAD(start_broadcasting_task(struct pt *pt, pbio_task_t *task)) {
         is_broadcasting = true;
     }
 
-    // Delay to allow for advertising to start. FIXME: This is technically only
-    // needed if a previous broadcast was started sooner than this. It would be
-    // better to conditionally await just before the broadcast so we can avoid
-    // unnecessary delays.
-    timer_set(&broadcast_delay, 10);
-    PT_WAIT_UNTIL(pt, timer_expired(&broadcast_delay));
+    // Wait advertising enable command to complete.
+    PT_WAIT_UNTIL(pt, event_packet && HCI_EVENT_IS_COMMAND_COMPLETE(event_packet, hci_le_set_advertising_data));
 
-    // REVISIT: use callback to actually wait for start?
     task->status = PBIO_SUCCESS;
 
     PT_END(pt);