mmc: atmel-mci: map DMA sglist on the DMA engine

As established for the MMCI, it is proper to map the DMA buffers
on the DMA engine which is the one actually performing the DMA.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>