gsm0808: Fix encoding of the SAPI_N_REJECT BSSMAP message.

See TS 08.08 section 3.2.1.34 SAPI "n" REJECT:
1) DLCI is a TV element, not V.
2) Cause is a TLV element and we have a special function to encode it.

Change-Id: I033afe556c06427d06ac55c4f78854a45e41aae6
diff --git a/src/gsm/gsm0808.c b/src/gsm/gsm0808.c
index f9f7b58..02288e6 100644
--- a/src/gsm/gsm0808.c
+++ b/src/gsm/gsm0808.c
@@ -447,8 +447,8 @@
 		return NULL;
 
 	msgb_v_put(msg, BSS_MAP_MSG_SAPI_N_REJECT);
-	msgb_v_put(msg, link_id);
-	msgb_v_put(msg, GSM0808_CAUSE_BSS_NOT_EQUIPPED);
+	msgb_tv_put(msg, GSM0808_IE_DLCI, link_id);
+	gsm0808_enc_cause(msg, GSM0808_CAUSE_BSS_NOT_EQUIPPED);
 
 	msg->l3h = msgb_tv_push(msg, BSSAP_MSG_BSS_MANAGEMENT, msgb_length(msg));