[media] omap3isp: resizer: Improved resizer rsz factor formula

Round properly the rsz factor so that we get highest rsz so that the input
width (or height) is highest possible smaller or equal to what the user
asks.

Signed-off-by: Sakari Ailus <sakari.ailus@maxwell.research.nokia.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>