msm: camera: isp: Reduce the spinlock window

Reducing spin_lock_bh window during flush operation to
avoid spinlock recursion that may arise due to spin_unlock_bh
which is called within cam_sync_signal api.

Change-Id: I00d8d6d2d3ad0254233d7d15360fdda973520aee
Signed-off-by: Alok Pandey <akumarpa@codeaurora.org>