bssgp: Don't include RA-ID in BVC-RESET for BVCI=0 (signalling)

3GPP TS 48.018 is quite clear: The RA-ID must only be included
when a PTP-BVCI is being reset [and only if the sender is the BSS].

Before this patch, osmo-pcu is including the RA-ID in BVC-RESET
for BVCI=0.

Change-Id: Ie87820537d6d616da4fd4bbf73eab06e28fda5e1
diff --git a/src/gb/gprs_bssgp_bss.c b/src/gb/gprs_bssgp_bss.c
index 94b1812..d1734ee 100644
--- a/src/gb/gprs_bssgp_bss.c
+++ b/src/gb/gprs_bssgp_bss.c
@@ -318,7 +318,16 @@
 }
 int bssgp_tx_bvc_reset(struct bssgp_bvc_ctx *bctx, uint16_t bvci, uint8_t cause)
 {
-	return bssgp_tx_bvc_reset2(bctx, bvci, cause, bvci != BVCI_PTM);
+	/* The Cell Identifier IE is mandatory in the BVC-RESET PDU sent from BSS to SGSN in order to reset a
+	 * BVC corresponding to a PTP functional entity. The Cell Identifier IE shall not be used in any other
+	 * BVC-RESET PDU. */
+	switch (bvci) {
+	case BVCI_SIGNALLING:
+	case BVCI_PTM:
+		return bssgp_tx_bvc_reset2(bctx, bvci, cause, false);
+	default:
+		return bssgp_tx_bvc_reset2(bctx, bvci, cause, true);
+	}
 }
 
 /*! Transmit a FLOW_CONTROL-BVC (Chapter 10.4.4)