pcm: multi: return correct hwptr and avail from snd_pcm_multi_status()

[GitHub pull_request - opened <github@xxxxxxxxxxxxxxxx>]

alsa-project/alsa-lib pull request #224 was opened from infmagic2047:

Ensure the logic of snd_pcm_multi_status() is consistent with
snd_pcm_multi_avail_update().

Signed-off-by: Yutao Yuan <infmagic2047reg@xxxxxxxxxxx>

Request URL   : https://github.com/alsa-project/alsa-lib/pull/224
Patch URL     : https://github.com/alsa-project/alsa-lib/pull/224.patch
Repository URL: https://github.com/alsa-project/alsa-lib