ASoC: msm: set pointers to NULL after kfree

In lsm-related driver files, some pointers are not set as NULL
after the memory is freed, which will leave many dangling pointers.
Set them to NULL explicitly to avoid potential risk.

CRs-Fixed: 997059
Change-Id: I44925240705608510266a51225cc02611637c571
Signed-off-by: Walter Yang <yandongy@codeaurora.org>