USART_CR3_RTSE
cr3 |= USART_CR3_CTSE | USART_CR3_RTSE;
stm32_usart_set_bits(port, ofs->cr3, USART_CR3_RTSE);
stm32_usart_clr_bits(port, ofs->cr3, USART_CR3_RTSE);