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/osmo-bsc/osmo_bsc_msc.c b/openbsc/src/osmo-bsc/osmo_bsc_msc.c
index 55532db..a979681 100644
--- a/openbsc/src/osmo-bsc/osmo_bsc_msc.c
+++ b/openbsc/src/osmo-bsc/osmo_bsc_msc.c
@@ -494,5 +494,8 @@
 	msc_data->nr = nr;
 	msc_data->allow_emerg = 1;
 
+	/* Defaults for the audio setup */
+	msc_data->amr_conf.m5_90 = 1;
+
 	return msc_data;
 }