ALSA: ca0106: Use the standard snd_ctl_add_followers() helper

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

Also, handle the errors from the helper more properly instead of
silently ignoring.

Link: https://lore.kernel.org/r/20230721071643.3631-4-tiwai@suse.de


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