mmc: s3cmci: Use DMA slave map rather than exported DMA filter

Support for DMA slave map has been added to the s3c24xx-dma
controller in commit 34681d84


"dmaengine: s3c24xx: Add dma_slave_map for s3c2440 devices"
This patch converts the s3cmci driver to also use it, so we can
eventually get rid of the exported filter function once all
related DMA clients are updated.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>