mmc: atmel-mci: the r/w proof capability lack was not well managed

First mci IPs (mainly on rm9200 and 9261) don't have the r/w proof
capability.  The driver didn't work correctly without this capability
in PDC mode because of the double buffer switch which is too slow
even if we stop the transfer to perform this switch.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>