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.

platforms/chibios/drivers/audio_dac_additive.c

@@ -89,7 +89,7 @@ static dacsample_t dac_buffer_empty[AUDIO_DAC_BUFFER_SIZE] = {AUDIO_DAC_OFF_VALU
 /* keep track of the sample position for for each frequency */
 static float dac_if[AUDIO_MAX_SIMULTANEOUS_TONES] = {0.0};
 
-static float   active_tones_snapshot[AUDIO_MAX_SIMULTANEOUS_TONES] = {0, 0};
+static float   active_tones_snapshot[AUDIO_MAX_SIMULTANEOUS_TONES] = {0};
 static uint8_t active_tones_snapshot_length                        = 0;
 
 typedef enum {