[mgcp] Add a helper function to convert from GSM0808 ts/mux to MGCP endpoint

Move the conversion of GSM0808 timeslot and multiplex from
the bssap.c into the mgcp.h so it can be reused by multiple
users. The weird math comes from the mapping of the MSC...

openbsc/include/openbsc/mgcp.h

@@ -119,5 +119,13 @@ void mgcp_free_endp(struct mgcp_endpoint *endp);
 struct msgb *mgcp_handle_message(struct mgcp_config *cfg, struct msgb *msg);
 struct msgb *mgcp_create_response_with_data(int code, const char *msg, const char *trans, const char *data);
 
+/* adc helper */
+static inline int mgcp_timeslot_to_endpoint(int multiplex, int timeslot)
+{
+	if (timeslot == 0)
+		timeslot = 1;
+	return timeslot + (31 * multiplex);
+}
+
 
 #endif