at91_mci: Cover more AT91RM9200 and AT91SAM9261 errata.

According to the documentation the AT91SAM9261 MCI shares the block size
limitations of the AT91RM9200 MCI. Also the errata documentation for
AT91RM9200 and AT91SAM9261 state that stream commands are not supported.
This has not been tested on actual hardware.

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>