sgtl5000: change digital_mute policy

Current implementation mute codec in global way (DAC block).
That means when user routes sound not from I2S but from
AUX source (LINE_IN) it also will be muted by alsa core.
This should not happen.

Signed-off-by: Michal Oleszczyk <oleszczyk.m@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>