[ALSA] ice1724 - Add PCM Playback Switch to Revo 7.1

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: Pavel Hofman <dustin@seznam.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>