msm: camera: sync: Add to list when triggering callback

When sync object is already signaled and callback is registered,
trigger the callback, but before triggering make sure it is added
to the linked list.

Otherwise, after the triggering is done the
work queue will try to free the callback data which can cause
a crash. Adding to the linked list will avoid this crash.

Change-Id: Ic38b95288ad21a706a4f51a1b3f3d087e974ba1e
Signed-off-by: Seemanta Dutta <seemanta@codeaurora.org>