media: omap3isp: support ctrl events for isppreview

media: omap3isp: support ctrl events for isppreview

The preview subdev device was missing V4L2_SUBDEV_FL_HAS_EVENTS,
and that prevented VIDIOC_SUBSCRIBE_EVENT from working.

Fixes a v4l2-compliance error:

	fail: v4l2-test-controls.cpp(1128): subscribe event for control 'User Controls' failed
test VIDIOC_(UN)SUBSCRIBE_EVENT/DQEVENT: FAIL

Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>