drivers: flash: STM32WBA flash_stm32wba_fm.c: fix sector erase error

The STM32WBA Flash Manager driver is failing to erase a sector, instead of
erasing one sector, two sectors are erased.
Fix it by correctly calculating the number of sectors to erase

Signed-off-by: Eric Mechin <eric.mechin@st.com>

Author: ericmechin

drivers/flash/flash_stm32wba_fm.c

@@ -95,7 +95,7 @@ static int flash_stm32_erase(const struct device *dev, off_t offset,
 			     size_t len)
 {
 	int rc;
-	int sect_num = (len / FLASH_PAGE_SIZE) + 1;
+	int sect_num;
 
 	if (!flash_stm32_valid_range(dev, offset, len, true)) {
 		LOG_ERR("Erase range invalid. Offset: %p, len: %zu",
@@ -107,6 +107,9 @@ static int flash_stm32_erase(const struct device *dev, off_t offset,
 		return 0;
 	}
 
+	/* len is a multiple of FLASH_PAGE_SIZE */
+	sect_num = len / FLASH_PAGE_SIZE;
+
 	flash_stm32_sem_take(dev);
 
 	LOG_DBG("Erase offset: %p, page: %ld, len: %zu, sect num: %d",