[media] atmel-isi: setup the ISI_CFG2 register directly

In the function configure_geometry(), we will setup the ISI CFG2
according to the sensor output format.

It make no sense to just read back the CFG2 register and just set part
of it.

So just set up this register directly makes things simpler.
Currently only support YUV format from camera sensor.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Reviewed-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>