ALSA: hda - Add "Mute-LED Mode" enum control

Create snd_hda_add_vmaster_hook() and snd_hda_sync_vmaster_hook()
helper functions to handle the mute-LED in vmaster hook more
commonly.  In the former function, a new enum control "Mute-LED Mode"
is added.  This provides user to choose whether the mute-LED should be
turned on/off explicitly or to follow the master-mute status.

Reviewed-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>