[media] omap3isp: preview: Optimize parameters setup for the common case

If no parameter needs to be modified, make preview_config() and
preview_setup_hw() return immediately. This speeds up interrupt handling
in the common case.

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