ASoC: Fix NULL pointer dereference in digital suspend

If soundcard registration has failed because of custom
driver probe failure and at the same time
msm_dig_suspend is called, this will lead to
null pointer dereference when component is accessed.
Add NULL check to avoid this.

CRs-Fixed: 2263005
Change-Id: If04ec624e45f05ebe08ee6500ccb13e3c8287051
Signed-off-by: Vatsal Bucha <vbucha@codeaurora.org>