iio: adc: stm32-adc: make safe adc disable

From ADC reference manual the software is allowed to write the
control bit ADDIS of the ADC_CR register only if the ADC is enabled.
Return immediately from stm32h7_adc_disable() if ADC is already disabled.
Signed-off-by: NOlivier Moysan <olivier.moysan@foss.st.com>
Reviewed-by: NFabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20220620154738.801706-2-olivier.moysan@foss.st.comSigned-off-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/stm32-adc.c

@@ -876,6 +876,9 @@ static void stm32h7_adc_disable(struct iio_dev *indio_dev)
 	int ret;
 	u32 val;
 
+	if (!(stm32_adc_readl(adc, STM32H7_ADC_CR) & STM32H7_ADEN))
+		return;
+
 	/* Disable ADC and wait until it's effectively disabled */
 	stm32_adc_set_bits(adc, STM32H7_ADC_CR, STM32H7_ADDIS);
 	ret = stm32_adc_readl_poll_timeout(STM32H7_ADC_CR, val,