This seems more friendly to user-space, as it's notified at least as
an error, instead of forcibly moving the PCM state to SETUP out of
sudden.

Moreover, snd_pcm_stop() needs an extra PCM spinlock I forgot, while
snd_pcm_stop_xrun() takes the spinlock by itself.
Signed-off-by: Takashi Iwai <tiwai@suse.de>