prepare sgsn_mm_ctx for Gb and Iu mode (UMTS)

Explicitly mark those sgsn_mm_ctx members that apply for Gb mode and (upcoming)
Iu mode, respectively.

Add some comments in sgsn_mm_ctx.

Change-Id: Ife9b02549f284e2547f16117cf43d7a36948fc4b
Tweaked-By: Neels Hofmeyr <nhofmeyr@sysmocom.de>
diff --git a/openbsc/src/gprs/sgsn_cdr.c b/openbsc/src/gprs/sgsn_cdr.c
index d0cb712..bf0d6f7 100644
--- a/openbsc/src/gprs/sgsn_cdr.c
+++ b/openbsc/src/gprs/sgsn_cdr.c
@@ -94,7 +94,7 @@
 		mmctx->imsi,
 		mmctx->imei,
 		mmctx->msisdn,
-		mmctx->cell_id,
+		mmctx->gb.cell_id,
 		mmctx->ra.lac,
 		mmctx->hlr,
 		ev);
@@ -179,7 +179,7 @@
 		pdp->mm ? pdp->mm->imsi : "N/A",
 		pdp->mm ? pdp->mm->imei : "N/A",
 		pdp->mm ? pdp->mm->msisdn : "N/A",
-		pdp->mm ? pdp->mm->cell_id : -1,
+		pdp->mm ? pdp->mm->gb.cell_id : -1,
 		pdp->mm ? pdp->mm->ra.lac : -1,
 		pdp->mm ? pdp->mm->hlr : "N/A",
 		ev,