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.

components/driver/include/driver/uart.h

@@ -460,6 +460,19 @@ esp_err_t uart_set_rts(uart_port_t uart_num, int level);
  */
 esp_err_t uart_set_dtr(uart_port_t uart_num, int level);
 
+/**
+ * @brief Set UART idle interval after tx FIFO is empty
+ *
+ * @param uart_num UART_NUM_0, UART_NUM_1 or UART_NUM_2
+ * @param idle_num idle interval after tx FIFO is empty(unit: the time it takes to send one bit
+ *        under current baudrate)
+ *
+ * @return
+ *     - ESP_OK   Success
+ *     - ESP_FAIL Parameter error
+ */
+esp_err_t uart_set_tx_idle_num(uart_port_t uart_num, uint16_t idle_num);
+
 /**
 * @brief Set UART configuration parameters.
  *