msm: cpp: Fix potential dereference of NULL pointer

Work field in cpp_dev structure is not checked for NULL.
If its allocation fails, NULL pointer will be dereferenced.
This will cause crash in camera.

Change-Id: I8b36f3553292b41386e231f12867ff78f8e2379d
Signed-off-by: Iliya Varadzhakov <ivarad@codeaurora.org>