Software flow-control configuration
| SW_FLOW_CON_EN | Set this bit to enable software flow control. It is used with register sw_xon or sw_xoff. |
| XONOFF_DEL | Set this bit to remove flow control char from the received data. |
| FORCE_XON | Set this bit to enable the transmitter to go on sending data. |
| FORCE_XOFF | Set this bit to stop the transmitter from sending data. |
| SEND_XON | Set this bit to send Xon char. It is cleared by hardware automatically. |
| SEND_XOFF | Set this bit to send Xoff char. It is cleared by hardware automatically. |