mmc: atmel-mci: remove compat for non DT board when requesting dma chan

All boards with a dma controller have DT support so using
dma_request_slave_channel_compat is no more needed.

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