ALSA: usb-audio: set period_bytes in substream.

Set the peiod_bytes member of snd_usb_substream.  It was no longer being
set, but will be needed to resume properly in a future commit.
Signed-off-by: Dylan Reid <dgreid@chromium.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/pcm.c

@@ -486,6 +486,8 @@ static int snd_usb_hw_params(struct snd_pcm_substream *substream,
 	}
 
 	if (changed) {
+		subs->period_bytes = params_period_bytes(hw_params);
+
 		mutex_lock(&subs->stream->chip->shutdown_mutex);
 		/* format changed */
 		stop_endpoints(subs, 0, 0, 0);