mmc: block: fix format string warning

Fixed the following warning (reported by cppcheck):
[drivers/mmc/card/block.c:2149]: (warning) %d in format string (no. 1)
requires 'int' but the argument type is 'unsigned int'.
Signed-off-by: Asaf Vertz <asaf.vertz@tandemg.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/card/block.c

@@ -2147,7 +2147,7 @@ static struct mmc_blk_data *mmc_blk_alloc_req(struct mmc_card *card,
 	 */
 
 	snprintf(md->disk->disk_name, sizeof(md->disk->disk_name),
-		 "mmcblk%d%s", md->name_idx, subname ? subname : "");
+		 "mmcblk%u%s", md->name_idx, subname ? subname : "");
 
 	if (mmc_card_mmc(card))
 		blk_queue_logical_block_size(md->queue.queue,