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

Change-Id: I7f41a443f488b75df792597ec3cec8f7e97a7411
diff --git a/src/libosmo-mgcp/mgcp_stat.c b/src/libosmo-mgcp/mgcp_stat.c
index e63feac..acc9419 100644
--- a/src/libosmo-mgcp/mgcp_stat.c
+++ b/src/libosmo-mgcp/mgcp_stat.c
@@ -99,7 +99,7 @@
 	str += nchars;
 	str_len -= nchars;
 
-	if (conn->conn->endp->trunk->cfg->osmux != OSMUX_USAGE_OFF) {
+	if (conn->conn->endp->trunk->cfg->osmux_use != OSMUX_USAGE_OFF) {
 		/* Error Counter */
 		nchars = snprintf(str, str_len,
 				  "\r\nX-Osmo-CP: EC TI=%" PRIu64 ", TO=%" PRIu64,