msm: camera: core: Add missing list_del_init

Without list_del_init, this request will not be deleted from previous
list and also added to free list. This may result in double add.

Change-Id: I5503acb13260d01d72bd07bb2a7d6afe7ffc430d
Signed-off-by: Harsh Shah <harshs@codeaurora.org>