ASoC: msm: qdsp6v2: add check for NULL dereference

The asm loopback driver during close does not check
for NULL before closing the playback and capture
instances.

The change ensures only the valid playback/capture
instances are closed.

CRs-Fixed: 643724
Change-Id: I3373f62a2413e1497b6f8a401158444cd2870b2a
Signed-off-by: Aravind Kumar <akumark@codeaurora.org>