ALSA: vmaster: Add snd_ctl_add_followers() helper

Add a new helper to add multiple vmaster followers in a shot.  The
same function was open-coded in various places, and this helper
replaces them.

Link: https://lore.kernel.org/r/20230721071643.3631-2-tiwai@suse.de


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