iio: adc: stm32-dfsdm: fix sample rate for div2 spi clock

When channel clk source is set to "CLKOUT_F" or "CLKOUT_R" (e.g. div2),
sample rate is currently set to half the requested value.

Fixes: eca949800d2d ("IIO: ADC: add stm32 DFSDM support for PDM
microphone")

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/stm32-dfsdm-adc.c

@@ -771,7 +771,7 @@ static int stm32_dfsdm_write_raw(struct iio_dev *indio_dev,
 	struct stm32_dfsdm_adc *adc = iio_priv(indio_dev);
 	struct stm32_dfsdm_filter *fl = &adc->dfsdm->fl_list[adc->fl_id];
 	struct stm32_dfsdm_channel *ch = &adc->dfsdm->ch_list[chan->channel];
-	unsigned int spi_freq = adc->spi_freq;
+	unsigned int spi_freq;
 	int ret = -EINVAL;
 
 	switch (mask) {
@@ -785,8 +785,18 @@ static int stm32_dfsdm_write_raw(struct iio_dev *indio_dev,
 	case IIO_CHAN_INFO_SAMP_FREQ:
 		if (!val)
 			return -EINVAL;
-		if (ch->src != DFSDM_CHANNEL_SPI_CLOCK_EXTERNAL)
+
+		switch (ch->src) {
+		case DFSDM_CHANNEL_SPI_CLOCK_INTERNAL:
 			spi_freq = adc->dfsdm->spi_master_freq;
+			break;
+		case DFSDM_CHANNEL_SPI_CLOCK_INTERNAL_DIV2_FALLING:
+		case DFSDM_CHANNEL_SPI_CLOCK_INTERNAL_DIV2_RISING:
+			spi_freq = adc->dfsdm->spi_master_freq / 2;
+			break;
+		default:
+			spi_freq = adc->spi_freq;
+		}
 
 		if (spi_freq % val)
 			dev_warn(&indio_dev->dev,