USART_CR3_TXFTIE
cr3 &= USART_CR3_TXFTIE | USART_CR3_RXFTIE;
stm32_usart_set_bits(port, ofs->cr3, USART_CR3_TXFTIE);
stm32_usart_clr_bits(port, ofs->cr3, USART_CR3_TXFTIE);