pcm: use new APIs to add a control element set for softvol plugin
pcm: use new APIs to add a control element set for softvol plugin

In previous commit, some APIs to add a single element are discouraged to
continue using.

This commit replaces usage of the old APIs with new APIs.

Signed-off-by: Takashi Sakamoto <d5ec7d0cd8074757a1f5eb708bc105c3065a65ea@sakamocchi.jp>
Signed-off-by: Takashi Iwai <4596b3305151c7ee743192a95d394341e3d3b644@suse.de>

zedongchen/alsa-lib,alsa-project/alsa-lib,tiwai/alsa-lib,tiwai/alsa-lib,zedongchen/alsa-lib,zedongchen/alsa-lib,zedongchen/alsa-lib,takaswie/alsa-lib,alsa-project/alsa-lib,takaswie/alsa-lib,alsa-project/alsa-lib,tiwai/alsa-lib,takaswie/alsa-lib,takaswie/alsa-lib,tiwai/alsa-lib,takaswie/alsa-lib,zedongchen/alsa-lib,alsa-project/alsa-lib