R820t tuner fixes

These changes improve the R820T tuner code to calculate the VCO/PLL parameters more precisely. The old code rounded the reference frequency to the nearest kHz and did some slightly dodgy hand-rolled long division.

These changes empirically reduce the tuning error from approx +/- 750Hz to approx +/- 200Hz.
This is good enough to also fix some odd behaviour with applying frequency corrections, where for some PPM + frequency combinations, changing PPM by +/-1 would have no effect because the underlying tuner imprecision mapped both frequencies to the same PLL settings.

Tested on a generic RTL2832U with R820T tuner only.