[media] v4l: aptina-pll: Round up minimum multiplier factor value properly

The mf_low value must be a multiple of mf_inc. Round it up to the
nearest mf_inc multiple after computing it.

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>