[media] omap3isp: preview: Fix the crop margins

Commit 3fdfedaa "[media] omap3isp: preview: Lower the crop margins"
accidentally changed the previewer's cropping, causing the previewer
to miss four pixels on each line, thus corrupting the final image.
Restored the removed setting.

Cc: stable@vger.kernel.org
Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>