osmux: Rename field osmux usage policy and define it with proper type

Change-Id: I7f41a443f488b75df792597ec3cec8f7e97a7411
diff --git a/src/libosmo-mgcp/mgcp_osmux.c b/src/libosmo-mgcp/mgcp_osmux.c
index 5647348..06dc705 100644
--- a/src/libosmo-mgcp/mgcp_osmux.c
+++ b/src/libosmo-mgcp/mgcp_osmux.c
@@ -394,7 +394,7 @@
 
 	rate_ctr_inc(rate_ctr_group_get_ctr(all_rtp_stats, OSMUX_PACKETS_RX_CTR));
 
-	if (!trunk->cfg->osmux) {
+	if (trunk->cfg->osmux_use == OSMUX_USAGE_OFF) {
 		LOGP(DOSMUX, LOGL_ERROR,
 		     "bsc-nat wants to use Osmux but bsc did not request it\n");
 		goto out;