bugfix(uart): set default tx idle num

bugfix(uart): set default tx idle num

Reported from:
https://github.com/espressif/esp-idf/issues/703
https://github.com/espressif/esp-idf/issues/917
In uart driver we didn't change the default value of tx idle num, so there would be a delay after tx FIFO is empty.

1. Add API to set tx idle interval before next data transmission. (The UART hardware can add an interval after tx FIFO is empty).
2. Set default tx idle interval to zero.
3. Add hardware disable in uart driver delete function.