frequency_count_khz() is a bit too inaccurate

[bruelltuete]

frequency_count_khz() returns a frequency measurement of a given clock, by sdk-default, with an accuracy of 2 kHz. The interval value of 10 means 2 kHz, as per data sheet.

Unfortunately, measuring the frequency of clock_ref often (very often) gives me 12001 kHz. That is, counting cycles against itself is already off. That's not great... Should the default interval be 12, giving us 500 Hz? That would be appropriate for a function that returns kHz.