ASoC: msm: qdsp6v2: Clear ION handle after freeing

Once ion memory is freed ion handle should be set to
NULL to avoid double free or any unintended access.
This change clears the ION handle after freeing up
the memory.

Change-Id: I37d76fe2641c111fa4376eff8f7e45f6795414ca
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>