ALSA: usb-audio: Deprecate async_unlink option

The async unlink behavior has been working over years.  The option was
provided only as a workaround for 2.4.x kernel.  Let's get rid of it.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c
index 51a9aa3..d7382a5 100644
--- a/sound/usb/endpoint.c
+++ b/sound/usb/endpoint.c
@@ -523,7 +523,7 @@
 	if (!force && ep->chip->shutdown) /* to be sure... */
 		return -EBADFD;
 
-	async = !can_sleep && ep->chip->async_unlink;
+	async = !can_sleep;
 
 	clear_bit(EP_FLAG_RUNNING, &ep->flags);