FF-A: Error code should be 32-bit sized

Change-Id: I35a17d4801cdb9456a68495a248807aacb175e8d
Signed-off-by: J-Alves <joao.alves@arm.com>
diff --git a/src/api.c b/src/api.c
index 2667283..75a06b7 100644
--- a/src/api.c
+++ b/src/api.c
@@ -1,5 +1,5 @@
 /*
- * Copyright 2018 The Hafnium Authors.
+ * Copyright 2021 The Hafnium Authors.
  *
  * Use of this source code is governed by a BSD-style
  * license that can be found in the LICENSE file or at
@@ -1549,7 +1549,7 @@
 		      "Minor revision representation takes more than 16 bits.");
 	if (requested_version & FFA_VERSION_RESERVED_BIT) {
 		/* Invalid encoding, return an error. */
-		return (struct ffa_value){.func = FFA_NOT_SUPPORTED};
+		return (struct ffa_value){.func = (uint32_t)FFA_NOT_SUPPORTED};
 	}
 
 	return (struct ffa_value){