dma: fix build breakage in s3c24xx-dma

This driver missed the dma unmap conversion.  Replace
s3c24xx_dma_unmap_buffers with dma_descriptor_unmap.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>