osmux: encapsulate for osmux state information in struct mgcp_endpoint

Just a cleanup, wrap around the osmux state information in a struct.
diff --git a/openbsc/src/libmgcp/mgcp_protocol.c b/openbsc/src/libmgcp/mgcp_protocol.c
index 1198c24..0989cc6 100644
--- a/openbsc/src/libmgcp/mgcp_protocol.c
+++ b/openbsc/src/libmgcp/mgcp_protocol.c
@@ -327,7 +327,8 @@
 	len = snprintf(sdp_record, sizeof(sdp_record),
 		       "I: %u%s\n\n",
 			endp->ci,
-			endp->cfg->osmux && endp->osmux ? "\nX-Osmux: On" : "");
+			endp->cfg->osmux && endp->osmux.enable ?
+				"\nX-Osmux: On" : "");
 
 	if (len < 0)
 		return NULL;
@@ -346,7 +347,7 @@
 
 static void send_dummy(struct mgcp_endpoint *endp)
 {
-	if (endp->osmux)
+	if (endp->osmux.enable)
 		osmux_send_dummy(endp);
 	else
 		mgcp_send_dummy(endp);