ASoC: msm: msm8994: add mixer command for SLIM0 TX bit format

As part of effort to support 24-bit recording on MSM8994, add
mixer control so user-space can choose bit format of SLIMBUS_0_TX
back-end through mixer control.

Change-Id: I9395ce4bfd56a3a48c8a86ab403b6f433ef0e626
Signed-off-by: Patrick Lai <plai@codeaurora.org>