msm: camera: isp: Protect flush from spinlock recursion

Flush can be called from user thread or CRM workqueue. Flush
takes the lock for ctx without disabling bottom halves. In
bottom half, event handling function we take the same lock
causing spinlock recursion. Change to spin_lock_bh to avoid
this.

Change-Id: I71a0f71bf4eb8b8f5d5eaff8a936a2f9804f8541
Signed-off-by: Harsh Shah <harshs@codeaurora.org>