SPM: Fix conversion issues on cactus responses

The checks of cactus responses in TFTF spm tests were failing to detect
CACTUS_ERROR, which is used to notify that some tests went wrong.
Thus, tests were being logged as finalized with success, when that was
not the case.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: Id4d915453cd137c9a4a2ec0072c9674f5c5fe0e5
diff --git a/spm/cactus/cactus_test_cmds.h b/spm/cactus/cactus_test_cmds.h
index 6329b6d..1fc4bba 100644
--- a/spm/cactus/cactus_test_cmds.h
+++ b/spm/cactus/cactus_test_cmds.h
@@ -13,8 +13,8 @@
 /**
  * Success and error return to be sent over a msg response.
  */
-#define CACTUS_SUCCESS	 0
-#define CACTUS_ERROR	-1
+#define CACTUS_SUCCESS	 U(0)
+#define CACTUS_ERROR	 U(-1)
 
 /**
  * Get command from struct smc_ret_values.