Allow the user to select one tone for the additive DAC (#21591)

Previously, if the user tried to use `#define AUDIO_MAX_SIMULTANEOUS_TONES 1` to conserve CPU time, their firmware would fail to build with this error:

```
platforms/chibios/drivers/audio_dac_additive.c:91:73: error: excess elements in array initializer [-Werror]
   91 | static float   active_tones_snapshot[AUDIO_MAX_SIMULTANEOUS_TONES] = {0, 0};
      |                                                                          ^
```

This also affected the preset `#define AUDIO_DAC_QUALITY_VERY_HIGH`, which called up only one simultaneous tone at 88200 Hz.

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