iio: adc: stm32: manage min sampling time on all internal channels

Force minimum sampling time for all internal channels according
to datasheet requirement. This value can be increased through
DT st,min-sample-time-ns property.

Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20221012142205.13041-5-olivier.moysan@foss.st.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>