audio: Make the BSC handle the new mr_config request of the BSC API

Handle the mr_config request and set the AMR multirate config for
the given MSC. Initialize the mr_config with the AMR5.9 default we
have been using until now.
diff --git a/openbsc/src/libbsc/bsc_api.c b/openbsc/src/libbsc/bsc_api.c
index ad89fb2..2fab20b 100644
--- a/openbsc/src/libbsc/bsc_api.c
+++ b/openbsc/src/libbsc/bsc_api.c
@@ -155,6 +155,12 @@
 static void handle_mr_config(struct gsm_subscriber_connection *conn,
 			     struct gsm_lchan *lchan)
 {
+	struct bsc_api *api;
+	api = conn->bts->network->bsc_api;
+
+	if (api->mr_config)
+		return api->mr_config(conn, &lchan->mr_conf);
+
 	lchan->mr_conf.ver = 1;
 	lchan->mr_conf.icmi = 1;
 	lchan->mr_conf.m5_90 = 1;