ALSA: ac97: Use the standard snd_ctl_add_followers() helper

Instead of open-code, use the new standard helper to manage vmaster
stuff for code simplification.

Except for a debug print, there should be no functional change.

Link: https://lore.kernel.org/r/20230721071643.3631-3-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>