media: common: saa7146: use for_each_sg_dma_page

When building the pgtables, use for_each_sg_dma_page.

Also clean up the code a bit.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>