[media] omap3isp: Fix division by 0

If the requested clock rate passed to the XCLK set_rate or round_rate
operation is 0, the driver will try to divide by 0. Fix this.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>