ASoC: msm: qdspv2: add spin lock to protect ac

ac could get freed during the execution of q6asm_callback.
And kernel panic happens. Add spinlock to protect ac to avoid
kernel panic.

Change-Id: Ie49c8a3979231552ba7d5f207aab0d95ffdc2a72
Signed-off-by: Meng Wang <mwang@codeaurora.org>