Error Checking Fixes

Fixes:

• Assertion error when executing si.clock_2.r_divider = R_DIV_128
• Assertion error when fractional pll denominator is 0xFFFFF

To use in CircuitPython, simply install the Adafruit CircuitPython bundle.

To use in CPython, pip install adafruit-circuitpython-si5351.

Read the docs for info on how to use it.