Platform: Minor warnings cleanup for IAR

Added cast to int32_t to eliminate warning for comparing unsigned
against < 0

Signed-off-by: TTornblom <thomas.tornblom@iar.com>
Change-Id: Iecfc2046c9b6d38b3a1a2aa2fead816ded03e3d4
diff --git a/secure_fw/partitions/platform/platform_sp.c b/secure_fw/partitions/platform/platform_sp.c
index 616032d..70fbf1a 100644
--- a/secure_fw/partitions/platform/platform_sp.c
+++ b/secure_fw/partitions/platform/platform_sp.c
@@ -50,7 +50,7 @@
 
     /* Boundary check the input argument */
     if (nv_counter_no >= NV_COUNTER_MAP_SIZE ||
-        nv_counter_no < 0 || nv_counter_no >= PLAT_NV_COUNTER_MAX) {
+        (int32_t)nv_counter_no < 0 || nv_counter_no >= PLAT_NV_COUNTER_MAX) {
         return false;
     }