ASoC: msm8x16: add mixer control to set BT sample rate

The interface to set sample rate for BT is not available. Its
set at 8KHz hardcoded and hence need mixer control to allow
set from user space based on need.

Change-Id: I8a522c9d96cff5a0433fce0a5b549b5b7f90a5fe
CRs-Fixed: 681676
Signed-off-by: Asish Bhattacharya <asishb@codeaurora.org>