mmc: at91/avr32/atmel-mci: fix DMA-channel leak on module unload

Fix regression introduced by commit 796211b7 ("mmc: atmel-mci: add
pdc support and runtime capabilities detection") which removed the need
for CONFIG_MMC_ATMELMCI_DMA but kept the Kconfig-entry as well as the
compile guards around dma_release_channel() in remove(). Consequently,
DMA is always enabled (if supported), but the DMA-channel is not
released on module unload unless the DMA-config option is selected.

Remove the no longer used CONFIG_MMC_ATMELMCI_DMA option completely.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Chris Ball <cjb@laptop.org>