gsm0808_create_cipher_reject: Fix encoding of Cause IE

The Cause IE in the 08.08 CIPHER MODE REJECT is a normal TLV IE,
and not just a value.  Let's make sure we encode the cause value
properly.

Change-Id: I4f5b231edf6dcb0a9c2bbafb2a59f301f3b2402b
Closes: OS#2766
diff --git a/src/gsm/gsm0808.c b/src/gsm/gsm0808.c
index 2396a10..abf3d25 100644
--- a/src/gsm/gsm0808.c
+++ b/src/gsm/gsm0808.c
@@ -240,7 +240,8 @@
 	if (!msg)
 		return NULL;
 
-	msgb_tv_put(msg, BSS_MAP_MSG_CIPHER_MODE_REJECT, cause);
+	msgb_v_put(msg, BSS_MAP_MSG_CIPHER_MODE_REJECT);
+	msgb_tlv_put(msg, GSM0808_IE_CAUSE, 1, &cause);
 
 	msg->l3h = msgb_tv_push(msg, BSSAP_MSG_BSS_MANAGEMENT, msgb_length(msg));