[ALSA] Fix substream to check in PCM drain

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: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>