ASoC: sti: Use snd_pcm_stop_xrun() helper

The XRUN trigger from the driver should be done via
snd_pcm_stop_xrun().  It fixes the missing stream locking as a gratis,
too.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mark Brown <broonie@kernel.org>