msm: camera: sync: delete cb from list before queue work

Delete cb list before adding work to workqueue, so that cb structure
will not be freed by other function when it is inside workqueue. cb
will free itself after dispatching.

Change-Id: I718c1c7a35603edc847043fdde272a4b407e824c
Signed-off-by: Junzhe Zou <jnzhezou@codeaurora.org>