PDC_RCR
bus_space_write_4(iot, ioh, offset + PDC_RCR, 0);
PUTREG(sc, SPI_PDC_BASE + PDC_RCR, 0);
at91usart_writereg(sc, US_PDC + PDC_RCR, 0);
at91usart_writereg(sc, US_PDC + PDC_RCR, 0); \