The missing barriers caused uart_get_buffered_data_len() to (very rarely)
return a garbage value. When used in MicroPython, though, this caused
select() to return and a subsequent read() to stall indefinitely until
a char was actually available.
Signed-off-by: Chen Yi Qun <chenyiqun@espressif.com>
Closes https://github.com/espressif/esp-idf/issues/6397
Merges https://github.com/espressif/esp-idf/pull/6396
e41e67f2f1