mmc: Ensure hardware partitions don't mess with mmcblk device naming.

With the hardware partitions support (which represent additional logical
devices present on MMC), devidx does not correspond with index used to form
/dev/mmcblkX names. So use an additional allocated index for device names.

Signed-off-by: Andrei Warkentin <andreiw@motorola.com>
Signed-off-by: Chris Ball <cjb@laptop.org>