bsc_api: Do not free the subscriber conn on clear and clear request

Do not free the GSM Subscriber Connection when a channel is failing
or if a clear is requested, instead just give up _all_ the channels,
reset them to NULL and free the remaining channels.
diff --git a/openbsc/src/gsm_04_08.c b/openbsc/src/gsm_04_08.c
index f022c04..b451be7 100644
--- a/openbsc/src/gsm_04_08.c
+++ b/openbsc/src/gsm_04_08.c
@@ -326,6 +326,8 @@
 		if (trans->conn == conn)
 			trans_free(trans);
 	}
+
+	subscr_con_free(conn);
 }
 
 /* Chapter 9.2.14 : Send LOCATION UPDATING REJECT */