iio: adc: stm32: fix a current leak by resetting pcsel before disabling vdda

Some I/Os are connected to ADC input channels, when the corresponding bit
in PCSEL register are set on STM32H7 and STM32MP15. This is done in the
prepare routine of stm32-adc driver.
There are constraints here, as PCSEL shouldn't be set when VDDA supply
is disabled. Enabling/disabling of VDDA supply in done via stm32-adc-core
runtime PM routines (before/after ADC is enabled/disabled).

Currently, PCSEL remains set when disabling ADC. Later on, PM runtime
can disable the VDDA supply. This creates some conditions on I/Os that
can start to leak current.
So PCSEL needs to be cleared when disabling the ADC.

Fixes: 95e339b6e8 ("iio: adc: stm32: add support for STM32H7")
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Reviewed-by: Olivier Moysan <olivier.moysan@foss.st.com>
Link: https://lore.kernel.org/r/1634905169-23762-1-git-send-email-fabrice.gasnier@foss.st.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/stm32-adc.c

@@ -1117,6 +1117,7 @@ static void stm32h7_adc_unprepare(struct iio_dev *indio_dev)
 {
 	struct stm32_adc *adc = iio_priv(indio_dev);
 
+	stm32_adc_writel(adc, STM32H7_ADC_PCSEL, 0);
 	stm32h7_adc_disable(indio_dev);
 	stm32_adc_int_ch_disable(adc);
 	stm32h7_adc_enter_pwr_down(adc);