drivers: io_stm32image: fix build warning on uninitialized variable

Fixes implementation against build warning reported by GCC:

drivers/st/io/io_stm32image.c: In function ‘stm32image_partition_read’:
drivers/st/io/io_stm32image.c:249:6: error: ‘result’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  int result;
      ^~~~~~

Actually, by construction the current implementation of function
stm32image_partition_read() does not mandate result to be initialized
since it always reaches the exit point with a valid value in 'result'.
Yet, this change prevents compiler from complaining and is more robust
against future changes in the implementation.

Change-Id: I383575edb605b7535398952a5fdfc266c0068c71
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>
Signed-off-by: Yann Gautier <yann.gautier@st.com>

drivers/st/io/io_stm32image.c

@@ -246,7 +246,7 @@ static int stm32image_partition_size(io_entity_t *entity, size_t *length)
 static int stm32image_partition_read(io_entity_t *entity, uintptr_t buffer,
 				     size_t length, size_t *length_read)
 {
-	int result;
+	int result = -EINVAL;
 	uint8_t *local_buffer;
 	boot_api_image_header_t *header =
 		(boot_api_image_header_t *)first_lba_buffer;