Add four mute controls for the analog output channels for cards that use
an SPI DAC, like the SB0570 SB Live! 24-bit / Audigy SE.  The Wolfson DAC
doesn't support muting left/right so the controls are mono.
The chip state struct gets a 32-byte array to act as a shadow of the spi
dac registers.  Only two registers are used for mute, but more would be
needed for analog gain, de-emphasis, DAC power down, phase inversion, and
other features.
Signed-off-by: NTrent Piepho <xyzzy@speakeasy.org>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>
Signed-off-by: NJaroslav Kysela <perex@suse.cz>