VFF_THRE
mtk_uart_apdma_write(c, VFF_THRE, VFF_TX_THRE(vff_sz));
mtk_uart_apdma_write(c, VFF_THRE, VFF_RX_THRE(vff_sz));
mtk_uart_apdma_write(c, VFF_THRE, 0);