This patch adds the support of mute for front channels of M-Audio
Revolution 7.1 (the DAC AK4381 features a mute bit).
Signed-off-by: NPavel Hofman <dustin@seznam.cz>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>
Signed-off-by: NJaroslav Kysela <perex@suse.cz>