msm: camera: reqmgr: Get the index difference properly

The slot index is a cycled index, need to add the
num slots and do complementation if we want to get
difference.

Change-Id: I01a7d0d7a79ad7beea95a6ed0e259b44879ab7c3
Signed-off-by: Depeng Shao <depengs@codeaurora.org>