Fix issue #22: VoltageMonitoring: wrong bit written to the AD7888's control register

Closes #22 (closed)

I changed the data sent to the SPI master to {2'b0, WordCount_b4[2:0], 3'b000, 8'h00} , because bits 2:0 of WordCount_b4 carry the channel number, while bit 3 is there to signalize that all the channels are read. Thus, bit 6 written to the control register is 0.