MMC: S3C24XX: Fix s3c2410_dma_request() return code check.

The driver should be checking for a negative error code from
s3c2410_dma_request(), not non-zero. Newer kernels now return
the DMA channel number that was allocated by the request.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>