MMC: Always use a sector size of 512 bytes

Both MMC and SD specifications specify (although a bit unclearly in the MMC
case) that a sector size of 512 bytes must always be supported by the card.

Cards can report larger "native" size than this, and cards >= 2 GB even
must do so. Most other readers use 512 bytes even for these cards. We should
do the same to be compatible.

Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>
Signed-off-by: Adrian Bunk <bunk@stusta.de>