[media] videobuf-dma-sg: Use min_t(size_t, PAGE_SIZE ..)

As pointed by Laurent:

I think min_t(size_t, PAGE_SIZE, size) is the preferred way.

Thanks-to: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>