Interface: Improve code quality

This patch fixes the following things:
* avoid implicit casting by using matching types or casting when it is
safe
* initialise unitialised variables
* check the returning value of functions

Change-Id: I0fda9b6d3ba9dbc86654685736a37a60f5db9a75
Signed-off-by: Hugues de Valon <hugues.devalon@arm.com>
diff --git a/interface/include/tfm_ns_svc.h b/interface/include/tfm_ns_svc.h
index 0399af9..a45cd97 100644
--- a/interface/include/tfm_ns_svc.h
+++ b/interface/include/tfm_ns_svc.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017-2018, Arm Limited. All rights reserved.
+ * Copyright (c) 2017-2019, Arm Limited. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  *
@@ -59,7 +59,7 @@
 };
 
 /* number of user SVC functions */
-#define USER_SVC_COUNT (SVC_TFM_MAX - 1)
+#define USER_SVC_COUNT ((uint32_t)SVC_TFM_MAX - 1)
 
 #ifdef __cplusplus
 }