在数据手册中有这样一段话：19.2.4 SPI Enable：
The SPI transmit/receive enable bit USPIEx enables or disables the USART in SPI mode. When USPIEx
= 0, the USART stops operation after the current transfer completes, or immediately if no operation is
active. A PUC or set SWRST bit disables the USART immediately and any active transfer is terminated.
可是如何设置USPIE=1，从而使得SPI模块工作呢？
