ALSA: hda: generic: Add vmaster mute LED helper

ALSA: hda: generic: Add vmaster mute LED helper

Like mic-mute LED handling, add a new helper to deal with the master
mute LED with LED classdev.  Unlike the mic-mute case, the playback
master mute is hooked on vmaster, and we suppose no nested hooks
allowed there.

The classdev creation code is factored out to a common function that
is called from both mute and mic-mute LED helpers.

Tested-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Link: https://lore.kernel.org/r/20200618110842.27238-9-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>