mmc: atmel: get rid of struct mci_dma_data

As struct mci_dma_data is now only used by AVR32, it is nothing but
pointless indirection.  Replace it with struct dw_dma_slave in the
AVR32 platform code and with a void pointer elsewhere.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Acked-by: Hans-Christian Noren Egtvedt <egtvedt@samfundet.no>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>