fix(mtd): correct types in messages

Some messages don't use the correct types, update them.
This avoids warning when -Wformat-signedness is enabled.

Change-Id: Ie5384a7d139c48a623e1617c93d15fecc8a36061
Signed-off-by: Yann Gautier <yann.gautier@st.com>
diff --git a/drivers/mtd/nor/spi_nor.c b/drivers/mtd/nor/spi_nor.c
index 6b4643e..2e34344 100644
--- a/drivers/mtd/nor/spi_nor.c
+++ b/drivers/mtd/nor/spi_nor.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2019-2021, STMicroelectronics - All Rights Reserved
+ * Copyright (c) 2019-2022, STMicroelectronics - All Rights Reserved
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -284,7 +284,7 @@
 	nor_dev.read_op.addr.val = offset;
 	nor_dev.read_op.data.buf = (void *)buffer;
 
-	VERBOSE("%s offset %i length %zu\n", __func__, offset, length);
+	VERBOSE("%s offset %u length %zu\n", __func__, offset, length);
 
 	while (length != 0U) {
 		if ((nor_dev.flags & SPI_NOR_USE_BANK) != 0U) {