mmc: atmel-mci: fix timeout errors in SDIO mode when using DMA

With some SDIO devices, timeout errors can happen when reading data. To solve
this issue the DMA transfer has to be activated before sending the command to
the device. This order is incorrect in PDC mode. So we have to take care if we
are using DMA or PDC to know when to send the MMC command.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>