Restore MT63 cursor

Force MT63 wf cursor to fixed tracking points; 750, 1000, 1500 for
MT63-500/1000/2000 respectively

src/mt63/mt63.cxx

@@ -117,6 +117,8 @@ int mt63::rx_process(const double *buf, int len)
 	unsigned int c;
 	int i;
 
+	if (get_freq() != (500.0 + bandwidth / 2.0)) 	set_freq(500.0 + bandwidth / 2.0);
+
 	if (Interleave != progdefaults.mt63_interleave) {
 		Interleave = progdefaults.mt63_interleave;
 		restart();