[media] atmel-isi: remove mck backward compatibility code

The master clock should be handled by sensor itself.

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