The check of a substream in snd_pcm_drain() might not be always correct
since runtime can point a different substream (although the PCM state
of each linked substream should be same, in theory).
This patch fixes it.
Signed-off-by: NTakashi Iwai <tiwai@suse.de>
Signed-off-by: NJaroslav Kysela <perex@suse.cz>