lchan: Create a structure for MSC data of the lchan

Prepare to split the BSC and the MSC part by putting the
MSC data for a connection into a "gsm_subscriber_connection"
struct and renaming the macros.
diff --git a/openbsc/src/ussd.c b/openbsc/src/ussd.c
index a3d11f0..5476919 100644
--- a/openbsc/src/ussd.c
+++ b/openbsc/src/ussd.c
@@ -62,7 +62,7 @@
 /* A network-specific handler function */
 static int send_own_number(const struct msgb *msg, const struct ussd_request *req)
 {
-	char *own_number = msg->lchan->subscr->extension;
+	char *own_number = msg->lchan->conn.subscr->extension;
 	char response_string[GSM_EXTENSION_LENGTH + 20];
 
 	/* Need trailing CR as EOT character */