[MTD] mtd_blkdevs.c: Fix names when many devices/partitions are created

mtdblock (and other mtd modules that use the mtd_blkdevs interface
between the mtd translation layers and the linux block layer) handles
incorrectly more than 10 devices or 26 partitions in the names passed to
the generic disk layer.  This causes the device file names and other
info kept by the generic disk/block layers to have names such as
"mtdblock<".  Use integer formatting for device numbers; use "aa-az"
for partitions 27-52, "ba-bz" for 53-78...

Signed-off-by: Todd Poynor <tpoynor@mvista.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>