ALSA: pcm: allow for trigger_tstamp snapshot in .trigger

Don't use generic snapshot of trigger_tstamp if low-level driver or
hardware can get a more precise value for better audio/system time
synchronization.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

include/sound/pcm.h

@@ -285,6 +285,7 @@ struct snd_pcm_runtime {
 	/* -- Status -- */
 	struct snd_pcm_substream *trigger_master;
 	struct timespec trigger_tstamp;	/* trigger timestamp */
+	bool trigger_tstamp_latched;     /* trigger timestamp latched in low-level driver/hardware */
 	int overrange;
 	snd_pcm_uframes_t avail_max;
 	snd_pcm_uframes_t hw_ptr_base;	/* Position at buffer restart */