mmc: atmel-mci: abort transfer on timeout error

When a software timeout occurs, the transfer is not stopped. In DMA case,
it causes DMA channel to be stuck because the transfer is still active
causing following transfers to be queued but not computed.

Cc: <stable@vger.kernel.org> # 3.9+
Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Reported-by: Alexander Morozov <etesial@gmail.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>