Fix feedback divider value approximation

* The rounding algorithm here expects to start with the 2 digits
  after the decimal point in pll->feedbackDivFrac. Multiplying with
  1000 instead of 100 filled it with 3 digits.
  The last step then set the fractional part to zero because it was
  still too large.
Make code a bit easier to read, add some comments

* No functional change intended
Oops, remove leftover "TODO" in comment