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_vty.c b/openbsc/src/gprs/sgsn_vty.c
index f16c95a..02c0f31 100644
--- a/openbsc/src/gprs/sgsn_vty.c
+++ b/openbsc/src/gprs/sgsn_vty.c
@@ -431,12 +431,12 @@
 	vty_out(vty, "%sMM Context for IMSI %s, IMEI %s, P-TMSI %08x%s",
 		pfx, mm->imsi, mm->imei, mm->p_tmsi, VTY_NEWLINE);
 	vty_out(vty, "%s  MSISDN: %s, TLLI: %08x%s HLR: %s",
-		pfx, mm->msisdn, mm->tlli, mm->hlr, VTY_NEWLINE);
+		pfx, mm->msisdn, mm->gb.tlli, mm->hlr, VTY_NEWLINE);
 	vty_out(vty, "%s  MM State: %s, Routeing Area: %u-%u-%u-%u, "
 		"Cell ID: %u%s", pfx,
 		get_value_string(gprs_mm_st_strs, mm->mm_state),
 		mm->ra.mcc, mm->ra.mnc, mm->ra.lac, mm->ra.rac,
-		mm->cell_id, VTY_NEWLINE);
+		mm->gb.cell_id, VTY_NEWLINE);
 
 	vty_out_rate_ctr_group(vty, " ", mm->ctrg);