revert back spi.c to fix spi read issue (#933)
revert back spi.c to fix spi read issue (#933)


schelleg/PYNQ,cathalmccabe/PYNQ,Xilinx/PYNQ,cathalmccabe/PYNQ,cathalmccabe/PYNQ,Xilinx/PYNQ,cathalmccabe/PYNQ,yunqu/PYNQ,yunqu/PYNQ,Xilinx/PYNQ,schelleg/PYNQ,yunqu/PYNQ,schelleg/PYNQ,yunqu/PYNQ,Xilinx/PYNQ,schelleg/PYNQ,cathalmccabe/PYNQ,schelleg/PYNQ,cathalmccabe/PYNQ,yunqu/PYNQ,Xilinx/PYNQ