ASoC: codec: micb ref count decrement check

In SSR with recording, race condition which reduces micb_ref to -1
is seen. SSR sets micb_ref to 0, further recording widgets power
down can reduce it to -1. During power up, it is increased to 0,
micbias is not getting enabled for recording since micb_ref is 0.
To prevent this micb_ref is checked for non zero value before
decrementing it.

CRs-Fixed: 994268
Change-Id: I6ea23fdf8b119cfd178c4f5b79b9d1c01c267a82
Signed-off-by: dojha <dojha@codeaurora.org>