ASoC: msm: qdsp6v2: add support for device mute

Add support for muting the RX and TX devices without affecting
the voice stream. This feature is added to aid in voice call
quality testing.

CRs-Fixed: 634690
Change-Id: I6e4eeda853bb5aa31166d1b66f486142397a6b43
Signed-off-by: Shiv Maliyappanahalli <smaliyap@codeaurora.org>