msm: camera: reqmgr: return if sync_slot_idx is not found

While finding slot for the index if the slot is not found
the sync_slot_idx value is set to -1 and identified as
sync link is not ready, to avoid further checks return
with -EAGAIN once the value is set to -1.

Change-Id: Ibb22406dd27d45f139df8d03389b8833fbfe9614
Signed-off-by: Tejas Prajapati <tpraja@codeaurora.org>