msm: camera: isp: Avoid stop during IFE flush

This change avoids stopping hardware in IFE flush as the sequence
to stop HW is leading to bad HW state. This change also removes
BUG from irq controller.

Change-Id: Icf86c13c85370f32a4d37f02909ff0594943be59
Signed-off-by: Venkat Chinta <vchinta@codeaurora.org>