build/openocd: Disable polling after init

OpenOCD's event polling mechanism may interfere with
jtagstream, as we don't expect any command sent by
openocd itself.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>

litex/build/openocd.py

@@ -185,6 +185,7 @@ proc jtagstream_serve {tap port} {
         write_to_file("stream.cfg", cfg)
         script = "; ".join([
             "init",
+            "poll off",
             "irscan $_CHIPNAME.tap {:d}".format(ir),
             "jtagstream_serve $_CHIPNAME.tap {:d}".format(port),
             "exit",