mmc: atmel-mci: don't use dma features when using DMA with no chan available

Some callbacks are set too early -- i.e. we can have dma capabilities but
we can't get a dma channel. So wait to get the dma channel before setting
callbacks and change logs consequently.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
[Should be applied to 3.2-stable.]
Cc: <stable@vger.kernel.org>
Signed-off-by: Chris Ball <cjb@laptop.org>