mmc: Fix the block device read only flag

While the MMC handled the card's read only flag correctly on open,
it did not setup the flag in the allocated disk structure. The
consequence being that probing the /sys/class/block/mmcblkX/ro
attribute always reported 0.

Signed-off-by: Marc-Andre Hebert <hebert.marcandre@gmail.com>
Reviewed-by: Chris Ball <cjb@laptop.org>
Tested-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Chris Ball <cjb@laptop.org>