mtd: atmel_nand: fix typo in dev_err error message

Fix typo, "Unkown" -> "Unknown"
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Acked-by: NJosh Wu <josh.wu@atmel.com>
Signed-off-by: NBrian Norris <computersforpeace@gmail.com>

drivers/mtd/nand/atmel_nand.c

@@ -1719,7 +1719,7 @@ static int nfc_wait_interrupt(struct atmel_nand_host *host, u32 flag)
 		comp[index++] = &host->nfc->comp_cmd_done;
 
 	if (index == 0) {
-		dev_err(host->dev, "Unkown interrupt flag: 0x%08x\n", flag);
+		dev_err(host->dev, "Unknown interrupt flag: 0x%08x\n", flag);
 		return -EINVAL;
 	}