ASoC: wcd9306: add support for digital mute ops

Alsa ASoC framework provides mute function to avoid click
and pop in pcm prepare function. Change adds op for
digital mute function in codec driver which will be called
by the asoc frame work when audio usecase is enabled.

Change-Id: I067eec9abd1551281ddb92ded614492d115e3a6e
Signed-off-by: Aravind Kumar <akumark@codeaurora.org>