spi/s3c64xx: Merge dma control code

This patch modifies to merge the dma control code. Original
s3c64xx spi driver has each dma control code for rx and tx
channel. This patch merges these dma control codes into one.
With this patch, a dma setup function and callback function
handle for both rx and tx channel.

Signed-off-by: Boojin Kim <boojin.kim@samsung.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>