reset spread spectrum enable in init code

[wa2mze]

This is being added to the adafruit si5351 library
adafruit/Adafruit_Si5351_Library#16
There have been a few reported problems of Si5351 generating frequency outputs way to low or to high in frequency. This was reported to have been a fix. Perhaps the chip doesn't set this register to the correct default on power up under certain conditions.