Update cipher reject function

* add note about (yet) unsupported standard feature
* use enum constant instead of hex value

Change-Id: I5cc5fc1ae8d5474dbc8d3385a493adf6420a3c98
Related: OS#3187
diff --git a/tests/gsm0808/gsm0808_test.c b/tests/gsm0808/gsm0808_test.c
index 0b2794f..7111db9 100644
--- a/tests/gsm0808/gsm0808_test.c
+++ b/tests/gsm0808/gsm0808_test.c
@@ -248,7 +248,7 @@
 	struct msgb *msg;
 
 	printf("Testing creating Cipher Reject\n");
-	msg = gsm0808_create_cipher_reject(0x23);
+	msg = gsm0808_create_cipher_reject(GSM0808_CAUSE_CCCH_OVERLOAD);
 	VERIFY(msg, res, ARRAY_SIZE(res));
 	msgb_free(msg);
 }