[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>

include/sound/ak4xxx-adda.h

@@ -43,6 +43,7 @@ struct snd_ak4xxx_ops {
 struct snd_akm4xxx_dac_channel {
 	char *name;		/* mixer volume name */
 	unsigned int num_channels;
+	char *switch_name;		/* mixer switch*/
 };
 
 /* ADC labels and channels */