msm: camera: core: Reset last flush req id as part of acquire ctx

KMD maintains a list of fixed contexts that are re-circulated, initializing
the flush req id in acquire to avoid the uncertainty of doing it in release
since ctx object is only put back to free list when ref count reaches 0.

Change-Id: Ia3a82cb8aacfcf93c6caf851c1bf2e73f4b762f1
Signed-off-by: Junzhe Zou <jnzhezou@codeaurora.org>