US_IR_RXRDY
at91_usart_spi_writel(aus, IER, US_IR_RXRDY);
at91_usart_spi_writel(aus, IDR, US_IR_RXRDY);
at91_usart_spi_writel(aus, IER, US_IR_RXRDY);
return aus->status & US_IR_RXRDY;
at91_usart_spi_writel(aus, IDR, US_IR_OVRE | US_IR_RXRDY);
#define US_OVRE_RXRDY_IRQS (US_IR_OVRE | US_IR_RXRDY)