[ALSA] Fix hang-up at disconnection of usb-audio

Fix hang-up at disconnection of usb-audio devices while accessing PCM.
Don't handle PCM operations any more after shutdown flag is set.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>

sound/usb/usbaudio.c

@@ -1469,7 +1469,8 @@ static int snd_usb_hw_free(struct snd_pcm_substream *substream)
 	subs->cur_audiofmt = NULL;
 	subs->cur_rate = 0;
 	subs->period_bytes = 0;
-	release_substream_urbs(subs, 0);
+	if (!subs->stream->chip->shutdown)
+		release_substream_urbs(subs, 0);
 	return snd_pcm_free_vmalloc_buffer(substream);
 }