msm: camera: Add missing unsubscribe function in server ops.

The entry for unsubsribe_event function is missing from the server ops
table. Add this entry and point to the correct implementation of the
function.

Change-Id: I083213f546556723167a088e0c64c5a20f3295c0
CRs-Fixed: 399710
Signed-off-by: Kiran Kumar H N <hurlisal@codeaurora.org>