The usb-audio driver detects XRUN at its complete callback, but the
actual code to trigger PCM XRUN is commented out because it caused
deadlock in the past.  This patch revives the PCM trigger properly.
It resulted in more than just enabling snd_pcm_stop(), but it had to
deduce the PCM substream with proper NULL checks and holds the stream
lock around the call.

Signed-off-by: Takashi Iwai <tiwai@suse.de>