msm: synx: default user callback fix

Avoid race with enqueue and dequeue of user callback
data. This is necessary since the callback added in the
event queue could be dequeued and variable cleaned up
before wake up function is completed.

Change-Id: I70aa09d8fc88bcb6e7e4b7aebd0f854a9de075d0
Signed-off-by: Sumukh Hallymysore Ravindra <shallymy@codeaurora.org>