media: ov5640: correct comments for default VGA to avoid confusion

When OV5640 work at DVP mode, the default initialization settings
make it output 30 frames per second. But when it work at CSI-2 mode
the default link frequency will make it output 60 frames per second,
so correct the comments to make it more clear.

Signed-off-by: Guoniu.zhou <guoniu.zhou@nxp.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>