V4L/DVB: buf-dma-sg.c: don't assume nr_pages == sglen

videobuf_pages_to_sg() and videobuf_vmalloc_to_sg() happen to create
a scatterlist element for every page.  However, this is not true for
bus addresses, so other functions shouldn't rely on the length of the
scatter list being equal to nr_pages.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>