ALSA: pcm: add SNDRV_PCM_TRIGGER_DRAIN trigger

Add SNDRV_PCM_TRIGGER_DRAIN trigger for pcm drain.

Some audio devices require notification of drain events
in order to properly drain and shutdown an audio stream.

Signed-off-by: Libin Yang <libin.yang@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --cc include/sound/pcm.h
Simple merge

diff --cc include/uapi/sound/asound.h
Simple merge

diff --cc sound/core/pcm_native.c
Simple merge