Interface: Improve code quality

This patch fixes implicit casting in the tfm_ns_lock_dispatch function.

Change-Id: I9f3db6e850f22c608c26b45a481cb3c910d2abd6
Signed-off-by: Hugues de Valon <hugues.devalon@arm.com>
diff --git a/interface/src/tfm_ns_lock_cmsis_rtos.c b/interface/src/tfm_ns_lock_cmsis_rtos.c
index a81f6e1..3747319 100644
--- a/interface/src/tfm_ns_lock_cmsis_rtos.c
+++ b/interface/src/tfm_ns_lock_cmsis_rtos.c
@@ -38,26 +38,26 @@
 /**
  * \brief NS world, NS lock based dispatcher
  */
-uint32_t tfm_ns_lock_dispatch(veneer_fn fn,
-                              uint32_t arg0, uint32_t arg1,
-                              uint32_t arg2, uint32_t arg3)
+int32_t tfm_ns_lock_dispatch(veneer_fn fn,
+                             uint32_t arg0, uint32_t arg1,
+                             uint32_t arg2, uint32_t arg3)
 {
-    uint32_t result;
+    int32_t result;
 
     /* Check the NS lock has been initialized */
     if (ns_lock.init == false) {
-        return TFM_ERROR_GENERIC;
+        return (int32_t)TFM_ERROR_GENERIC;
     }
 
     /* TFM request protected by NS lock */
     if (osMutexAcquire(ns_lock.id, osWaitForever) != osOK) {
-        return TFM_ERROR_GENERIC;
+        return (int32_t)TFM_ERROR_GENERIC;
     }
 
     result = fn(arg0, arg1, arg2, arg3);
 
     if (osMutexRelease(ns_lock.id) != osOK) {
-        return TFM_ERROR_GENERIC;
+        return (int32_t)TFM_ERROR_GENERIC;
     }
 
     return result;