Bluetooth: Audio: Fix broadcast sink PA sync callbacks register

The broadcast sink would continously register the PA sync callbacks,
effectively creating an infinite loop. Fixed by correctly
setting the boolean to prevent this.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>

subsys/bluetooth/audio/broadcast_sink.c

@@ -574,6 +574,8 @@ static void sync_broadcast_pa(const struct bt_le_scan_recv_info *info,
 		};
 
 		bt_le_per_adv_sync_cb_register(&cb);
+
+		pa_cb_registered = true;
 	}
 
 	sink = broadcast_sink_free_get();