mmc: atmel-mci: remove useless DMA stuff for non-dt devices

All devices with a DMA controller are DT compliant and legacy support
has been removed. For those reasons, some DMA stuff is useless.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>