msm: camera: reqmgr: Correct the check for lower pd devices

If no new req is pending in slot, finish the lower pipeline
delay devices with available req ids. There is possibility that
there is no lower pd devices on a given link and we will
modify the rd_idx & wr_idx incorrectly hence added validation
to avoid this.

Change-Id: Iaef5e72397ea6b341d4716127f1e013df63293b8
Signed-off-by: Karthik Anantha Ram <kartanan@codeaurora.org>