fix(st): add U suffix for unsigned numbers

This corrects MISRA c2012-7.2 violation:
A "u" or "U" suffix shall be applied to all integer constants that are
represented in an unsigned type.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I92b394572528e7179a314bbad4a032fd65053861
diff --git a/plat/st/common/bl2_io_storage.c b/plat/st/common/bl2_io_storage.c
index b271ed6..1bbaff6 100644
--- a/plat/st/common/bl2_io_storage.c
+++ b/plat/st/common/bl2_io_storage.c
@@ -607,7 +607,7 @@
  *     - we already boot FWU_MAX_TRIAL_REBOOT times in trial mode.
  * we select the previous_active_index.
  */
-#define INVALID_BOOT_IDX		0xFFFFFFFF
+#define INVALID_BOOT_IDX		0xFFFFFFFFU
 
 uint32_t plat_fwu_get_boot_idx(void)
 {