ASoC: sdm660_cdc: Add null check in msm_dig_suspend

In digital codec, null codec pointer access can happen
in suspend call if module register happen with pm_ops
but soc codec register failed for some reason. Add null
check in msm_dig_suspend API to avoid crash scenarios.

Change-Id: I15acb69a1851b7865d2b3aa1b7cb70a4a73c63e3
Signed-off-by: Laxminath Kasam <lkasam@codeaurora.org>