[media] v4l: atmel-isi: Should clear bits before set the hardware register

In the ISI driver it reads the config register to get original value,
then set the correct FRATE_DIV and YCC_SWAP_MODE directly. This will
cause some bits overlap.

So we need to clear these bits first, then set correct value. This patch
fix it.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>