Bug in lis2ds12_fifo_data_level_get implementation

Start at the lis2ds12_fifo_data_level_get implementation and consult the LIS2DS12 datasheet sections 8.24 and 8.25. Verify the FIFO sample count comes from the samples register rather than the threshold register, then check the driver’s available build or validation process to confirm the corrected behavior.

Current:

int32_t lis2ds12_fifo_data_level_get(stmdev_ctx_t *ctx, uint16_t *val)
{
  lis2ds12_fifo_ths_t fifo_ths;
  lis2ds12_fifo_src_t fifo_src;
  int32_t ret;

  ret = lis2ds12_read_reg(ctx, LIS2DS12_FIFO_THS, (uint8_t *)&fifo_ths, 1);

  if (ret == 0)
  {
    ret = lis2ds12_read_reg(ctx, LIS2DS12_FIFO_SRC, (uint8_t *)&fifo_src, 1);
    *val = fifo_src.diff;
    *val = (*val * 256U) +  fifo_ths.fth;
  }

  return ret;
}

Suggested:

int32_t lis2ds12_fifo_data_level_get(stmdev_ctx_t *ctx, uint16_t *val)
{
  uint8_t samples;
  lis2ds12_fifo_src_t fifo_src;
  int32_t ret;

  ret = lis2ds12_read_reg(ctx, LIS2DS12_FIFO_SAMPLES, &samples, 1);

  if (ret == 0)
  {
    ret = lis2ds12_read_reg(ctx, LIS2DS12_FIFO_SRC, (uint8_t *)&fifo_src, 1);
    *val = fifo_src.diff;
    *val = (*val * 256U) + samples;
  }

  return ret;
}

The problem with the current implementation is that it uses the LIS2DS12_FIFO_THS register to get the count of the samples in the FIFO. This is wrong because that register is the FIFO threshold. The function should be using LIS2DS12_FIFO_SAMPLES instead. This is all based on this datasheet, sections 8.24 and 8.25.