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/abis_rsl.c b/openbsc/src/abis_rsl.c
index e7844af..0c3b597 100644
--- a/openbsc/src/abis_rsl.c
+++ b/openbsc/src/abis_rsl.c
@@ -119,7 +119,7 @@
 
 	lchan = &ts->lchan[lch_idx];
 	debug_set_context(BSC_CTX_LCHAN, lchan);
-	debug_set_context(BSC_CTX_SUBSCR, lchan->subscr);
+	debug_set_context(BSC_CTX_SUBSCR, lchan->conn.subscr);
 
 	return lchan;
 }