msm: camera: core: Req list handling enhancement

On sync callback while flush add the req back into
free list to avoid leak. Add debug enhancement to
track each request moving among different lists
at different timelines which will help in debugging
how different requests are handled.

Change-Id: I56eb57ac74faf6b2c3d0d52b4437a4a5d34ef054
Signed-off-by: Pavan Kumar Chilamkurthi <pchilamk@codeaurora.org>