msm: camera: Reset last_flush_req on release

It is possible that context is released immediately after flush. In this
case last_flush_req does not reset to 0, so in next session all request
smaller then last_flush_req are rejected.

Change-Id: I44228efde88f5f1d1b90517162dbfd6874842574
Signed-off-by: Trishansh Bhardwaj <tbhardwa@codeaurora.org>