ASoC: wcd_cpe_core: Fix memory leak

During sound model de-registration, the message to be sent to CPE is
dynamically allocated but not freed. This causes memory leak in the
driver. Fix by adding kfree() to free the memory once the message
transfer is completed.

CRs-fixed: 933901
Change-Id: I02f25fc3457d530de436eb33da1709517b597e7b
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>