ALSA: Replace the word "slave" in vmaster API

Follow the recent inclusive terminology guidelines and replace the
word "slave" in vmaster API.  I chose the word "follower" at this time
since it seems fitting for the purpose.

Note that the word "master" is kept in API, since it refers rather to
audio master volume control.

Also, while we're at it, a typo in comments is corrected, too.

Link: https://lore.kernel.org/r/20200717154517.27599-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>