[media] media: vb2 dma-sg: Fully cache synchronise buffers in prepare and finish

In videobuf2 dma-sg memory types the prepare and finish ops, instead
of passing the number of entries in the original scatterlist as the
"nents" parameter to dma_sync_sg_for_device() and dma_sync_sg_for_cpu(),
the value returned by dma_map_sg() was used. Albeit this has been
suggested in comments of some implementations (which have since been
corrected), this is wrong.

Fixes: d790b7ed ("vb2-dma-sg: move dma_(un)map_sg here")

Cc: stable@vger.kernel.org # for v3.19 and up
Signed-off-by: Tiffany Lin <tiffany.lin@mediatek.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>