[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: Russell King <rmk+kernel@arm.linux.org.uk>