lpc176x移植包串口接收没有错误处理

lpc176x移植包串口接收没有错误处理，导致接收数据出错时中断无法清除。参照官方例程改动如下：

//
t-threadsplpc176xuart.c
void UART0_IRQHandler(void)
{
rt_ubase_t level, iir;
struct rt_uart_lpc* uart = &uart_device;

/* read IIR and clear it */
iir = LPC_UART->IIR;
iir >>= 1;                /* skip pending bit in IIR */
iir &= 0x07;            /* check bit 1~3, interrupt identification */
if (( iir == IIR_RLS && (LPC_UART0->LSR & LSR_RDR) ) || iir == IIR_RDA)        /* Receive Data Available */
{
    /* Receive Data Available */
    uart->rx_buffer[uart->save_index] = LPC_UART->RBR;
    level = rt_hw_interrupt_disable();
    uart->save_index ++;
    if (uart->save_index >= RT_UART_RX_BUFFER_SIZE)
        uart->save_index = 0;
    rt_hw_interrupt_enable(level);
    /* invoke callback */
    if(uart->parent.rx_indicate != RT_NULL)
    {
        rt_size_t length;
        if (uart->read_index > uart->save_index)
            length = RT_UART_RX_BUFFER_SIZE - uart->read_index + uart->save_index;
        else
            length = uart->save_index - uart->read_index;
        uart->parent.rx_indicate(&uart->parent, length);
    }
}
else if ( iir == IIR_RLS && (LPC_UART0->LSR & (LSR_OE|LSR_PE|LSR_FE|LSR_RXFE|LSR_BI)) )
{
    uart->rx_buffer[uart->save_index] = LPC_UART->RBR;    /*Dummy read on RX to clear interrupt*/
}
return;

}