ASoC: wcd9xxx: don't release mbhc firmware while it's still in use

When mbhc is de-initialized, driver shouldn't presume it's end of its
life as software reset might be happening so mbhc firmware should be
retained.  Move mbhc firmware release to mbhc exit function in order to
prevent improper firmware release.

CRs-fixed: 532512
Change-Id: I09b800a76d0502efae153cba9f2b0cc8365f8dce
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>