ASoC: msm8x16-wcd: 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.

CRs-Fixed: 662372
Change-Id: I1be21742e9027a15f77c6dd29c88582dbfdb3448
Signed-off-by: Aravind Kumar <akumark@codeaurora.org>