Remove 15 kHz from selectable flux ramp rates

[agustiner]

15 kHz does not divide 25.6 MHz and 122.88 MHz (?), which creates very sad looking flux ramp signals. All other frequencies, 1, 2, 3, 4, 5, 8, 10, 12 kHz are valid flux ramp rates from my testing so far. Right now users can apparently select any frequency desired with integer value.

Easy fix, remove 15 kHz, smarter fix, check the given flux ramp rate divides 25.6 Mhz and 122.88 MHz and then send it to the pyrogue layer.