[MMC] Improve MMC card block size selection

[MMC] Improve MMC card block size selection

Select a block size for IO based on the read and write block size
combinations, and whether the card supports partial block reads
and/or partial block writes.

If we are able to satisfy block reads but not block writes, mark
the device read only.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>