ASoc: msm8916: Add 16 bit PCM as default format for mi2s backend.

Add 16 bit PCM as default format for mi2s backend. This sets
backend format to be 16 bit pcm in case of offload playback.

Change-Id: Ic989b4c05b96688383bd703a101de1c4241bd420
Signed-off-by: Preetam Singh Ranawat <apranawat@codeaurora.org>