gsm0480: add specification reference to SS message type

Change-Id: Iff0210e995053e270939a774db33f55b22545204
diff --git a/src/gsm/gsm0480.c b/src/gsm/gsm0480.c
index 328a8c0..8319b19 100644
--- a/src/gsm/gsm0480.c
+++ b/src/gsm/gsm0480.c
@@ -285,6 +285,7 @@
 	int rc = 1;
 	uint8_t msg_type = hdr->msg_type & 0x3F;  /* message-type - section 3.4 */
 
+	/* Table 2.1: Messages for call independent SS control */
 	switch (msg_type) {
 	case GSM0480_MTYPE_RELEASE_COMPLETE:
 		LOGP(0, LOGL_DEBUG, "SS Release Complete\n");