ARM: S3C64XX: DMA: Protect buffer pointers while manipulation

Ensure the DMA buffer points are not updated from
another source during the process of enquing a buffer.

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