Store the GSM BTS in the gsm subscriber connection

Remove further usage of lchan from the gsm 04.11 bits
diff --git a/openbsc/include/openbsc/gsm_data.h b/openbsc/include/openbsc/gsm_data.h
index de69ee3..f352711 100644
--- a/openbsc/include/openbsc/gsm_data.h
+++ b/openbsc/include/openbsc/gsm_data.h
@@ -201,8 +201,9 @@
 	/* Are we part of a special "silent" call */
 	int silent_call;
 
-	/* back pointer to the gsm_lchan */
+	/* back pointers */
 	struct gsm_lchan *lchan;
+	struct gsm_bts *bts;
 };
 
 struct gsm_lchan {