[media] atmel-isi: increase timeout to disable/enable isi

If ISI is working on a 1024x768 or higher resolution, it needs longer
time to disable ISI. So this patch will increase timeout to 500ms.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>