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

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: Olivier Moysan <olivier.moysan@foss.st.com>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20220620154738.801706-2-olivier.moysan@foss.st.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>