fix(st): rework secure-status check in fdt_get_status()

This corrects MISRA C2012-15.7:
All if...else if constructs shall be terminated with an else statement.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I3d893d0db0a448323270086923563147008c59b9
diff --git a/plat/st/common/stm32mp_dt.c b/plat/st/common/stm32mp_dt.c
index 5a5ca56..6ec428c 100644
--- a/plat/st/common/stm32mp_dt.c
+++ b/plat/st/common/stm32mp_dt.c
@@ -79,11 +79,8 @@
 	}
 
 	cchar = fdt_getprop(fdt, node, "secure-status", NULL);
-	if (cchar == NULL) {
-		if (status == DT_NON_SECURE) {
-			status |= DT_SECURE;
-		}
-	} else if (strncmp(cchar, "okay", strlen("okay")) == 0) {
+	if (((cchar == NULL) && (status == DT_NON_SECURE)) ||
+	    ((cchar != NULL) && (strncmp(cchar, "okay", strlen("okay")) == 0))) {
 		status |= DT_SECURE;
 	}