ALSA: usb-audio: Don't start stream for capture at prepare

The recent change made mistakenly the stream for capture started at
prepare stage.  Add the stream direction check to avoid it.

Fixes: 9c9a3b9d ("ALSA: usb-audio: Rename early_playback_start flag with lowlatency_playback")
Link: https://lore.kernel.org/r/20211119102629.7476-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/pcm.c

@@ -640,7 +640,8 @@ static int snd_usb_pcm_prepare(struct snd_pcm_substream *substream)
 	runtime->delay = 0;
 
 	subs->lowlatency_playback = lowlatency_playback_available(runtime, subs);
-	if (!subs->lowlatency_playback)
+	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK &&
+	    !subs->lowlatency_playback)
 		ret = start_endpoints(subs);
 
  unlock: