gyro_fft: change default length 1024 -> 512 to decrease latency

- change default FFT length (1024 -> 512)
      - this doubles the update rate because half the number of samples are required for each
 - decrease number of peaks (4 -> 3)
      - so far 3 seems to be sufficient on most vehicles
 - increase median filter window (3 -> 5)
 - decrease SNR requirement (likely needs to be configurable)

src/modules/gyro_fft/parameters.c

@@ -66,10 +66,11 @@ PARAM_DEFINE_FLOAT(IMU_GYRO_FFT_MAX, 192.f);
 * IMU gyro FFT length.
 *
 * @value 256 256
+* @value 512 512
 * @value 1024 1024
 * @value 4096 4096
 * @unit Hz
 * @reboot_required true
 * @group Sensors
 */
-PARAM_DEFINE_INT32(IMU_GYRO_FFT_LEN, 1024);
+PARAM_DEFINE_INT32(IMU_GYRO_FFT_LEN, 512);