test(fft): update fft_dma example

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

sdk/software/examples/fft_dma/fft.h

@@ -10,6 +10,7 @@
 #define FFT_STAT_DONE     (1 << 1)
 #define FFT_STAT_BUSY     (1 << 0)
 #define FFT_POINT_NUM     1024
+#define FFT_SAMPLE_BITS    16
 
 void fft_start();
 void fft_wait();