tests/dac_dds: Fix sine wave to fit in PCM range

The previous sine wave cast signed integers into the PCM range, causing
jumps at zero transitions. This shifts everything up by the respective
maximum signed integer, so that the signed idle zero becomes the
unsigned PCM signal's idle half-point and can continuously cover the
whole unsigned range.

tests/driver_dac_dds/main.c

@@ -127,6 +127,7 @@ static void _fill_sine_samples_8(uint8_t *buf, size_t len, uint16_t period)
     for (uint16_t i = 0; i < period; ++i) {
         x += step;
         buf[i] = isin(x) >> 5;
+        buf[i] += INT8_MAX + 1;
     }
 
     for (uint16_t i = period; i < len; i += period) {
@@ -145,6 +146,7 @@ static void _fill_sine_samples_16(uint8_t *buf, size_t len, uint16_t period)
         x += step;
 
         uint16_t y = isin(x);
+        y += INT16_MAX + 1;
         buf[i]   = y;
         buf[++i] = y >> 8;
     }