Adjust chirp step to account for tone sample length

A chirp should cover the full tone scale in the length of a single beep. Two factors affect the chirp step per sample:

* The phase step for a particular frequency is scaled by the sample length multiplier.
* The length, in samples, of the beep is also scaled by the sample length multiplier.

Because of these two effects, ```Tone_chirp``` must be multiplied twice by ```TONE_SAMPLE_LEN``` to account for changes in the sample length.