Replace magic constants with macros.

Makes strlen_s limits clearer.

Change-Id: Iaa7488bdada389f5e50eea40091c541a360ae933
diff --git a/test/hftest/inc/hftest_impl.h b/test/hftest/inc/hftest_impl.h
index 34c73e6..346e496 100644
--- a/test/hftest/inc/hftest_impl.h
+++ b/test/hftest/inc/hftest_impl.h
@@ -272,7 +272,8 @@
 #define HFTEST_SERVICE_SELECT(vm_id, service, send_buffer)                    \
 	do {                                                                  \
 		struct hf_vcpu_run_return run_res;                            \
-		uint32_t msg_length = strnlen_s(service, 64);                 \
+		uint32_t msg_length =                                         \
+			strnlen_s(service, SERVICE_NAME_MAX_LENGTH);          \
                                                                               \
 		/*                                                            \
 		 * Let the service configure its mailbox and wait for a       \