at91_mci: AT91SAM9260/9263 12 byte write erratum (v2)

AT91SAM926[0/3] PDC must write at least 12 bytes. The code compiles and runs
but the actual condition for this erratum did not trigger in my tests so it's
unclear if it actually works as intended.

Signed-off-by: Ville Syrjala <syrjala@sci.fi>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>