cxp upconn: add packet type

src/libboard_artiq/src/cxp_upconn.rs

@@ -16,6 +16,7 @@ pub fn pipeline_test(timer: &mut GlobalTimer) {
     unsafe {
         // eop = 1 (End of packet) at the last data input
         csr::cxp::txcore_din_len_write(arr.len() as u8);
+        csr::cxp::txcore_packet_type_write(0x02); // read control command
 
         for a in arr.iter() {
             while csr::cxp::txcore_din_ready_read() == 0 {}