Clock resus does not fire correctly if external reference fails

[philipstarkey]

If the external reference fails, the clock resuscitation does not seem to fire correctly (or at least does not work the way I have coded it).

Not sure why. Need to investigate clocking configuration more anyway because the docs state that using resus to reconfigure the clocks is not recommended but I couldn't make clock reconfigurations work outside of forcing a resus.

[philipstarkey]

There were two compounding issues here.

1. I needed to disable the external pin after restoring the internal clock (even though there was no signal being fed into the external pin!)
2. I had a typo where I missed a * MHZ on the vco frequency.

I also removed the need to deinit the PLL to trigger the clock reconfigure (which was recommended against in the pico docs).

I'm including the fix for this in #10.