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/gprs_llc.c b/openbsc/src/gprs/gprs_llc.c
index 883f0cc..4d21963 100644
--- a/openbsc/src/gprs/gprs_llc.c
+++ b/openbsc/src/gprs/gprs_llc.c
@@ -56,8 +56,8 @@
 		dup.ms_ra_cap.v = mmctx->ms_radio_access_capa.buf;
 
 		/* make sure we only send it to the right llme */
-		OSMO_ASSERT(msgb_tlli(msg) == mmctx->llme->tlli
-				|| msgb_tlli(msg) == mmctx->llme->old_tlli);
+		OSMO_ASSERT(msgb_tlli(msg) == mmctx->gb.llme->tlli
+				|| msgb_tlli(msg) == mmctx->gb.llme->old_tlli);
 	}
 	memcpy(&dup.qos_profile, qos_profile_default,
 		sizeof(qos_profile_default));