MTD: Fix wrong check register_blkdev return value

register_blkdev return 1..255 when major = 0.

if (ret ) {
	printk(KERN_WARNING "Unable to register %s block device on major %d: %d\n",
		tr->name, tr->major, ret);
	      mutex_unlock(&mtd_table_mutex);
	return ret;
}

Above code will return fail when register_blkdev return allocated major number.

Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>