ARM: S3C64XX: DMA: Callback with correct buffer pointer

buffdone callback should be called per buffer request with pointer
to the latest serviced request.
'next' should point to the one next to currently active.

Signed-off-by: Jassi Brar <jassi.brar@samsung.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>