iio: adc: aspeed: Add divider flag to fix incorrect voltage reading.

commit 5714266 upstream.

The formula for the ADC sampling period in ast2400/ast2500 is:
ADC clock period = PCLK * 2 * (ADC0C[31:17] + 1) * (ADC0C[9:0])
When ADC0C[9:0] is set to 0 the sampling voltage will be lower than
expected, because the hardware may not have enough time to
charge/discharge to a stable voltage. This patch use the flag
CLK_DIVIDER_ONE_BASED which will use the raw value read from the
register, with the value of zero considered invalid to conform to the
corrected formula.

Fixes: 5738032 ("iio: Aspeed ADC")
Reported-by: Konstantin Klubnichkin <kitsok@yandex-team.ru>
Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20220221012705.22008-1-billy_tsai@aspeedtech.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/iio/adc/aspeed_adc.c

@@ -539,7 +539,9 @@ static int aspeed_adc_probe(struct platform_device *pdev)
 	data->clk_scaler = devm_clk_hw_register_divider(
 		&pdev->dev, clk_name, clk_parent_name, scaler_flags,
 		data->base + ASPEED_REG_CLOCK_CONTROL, 0,
-		data->model_data->scaler_bit_width, 0, &data->clk_lock);
+		data->model_data->scaler_bit_width,
+		data->model_data->need_prescaler ? CLK_DIVIDER_ONE_BASED : 0,
+		&data->clk_lock);
 	if (IS_ERR(data->clk_scaler))
 		return PTR_ERR(data->clk_scaler);