[ALSA] hda-codec - Don't create vmaster if no slaves found

Don't create vmaster controls if no slaves are found in the given list.
This prevents the error due to an empty vmaster control.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>