Fix channel init for ADC generic channel bindings.
In generic channel initialization, stm32_adc_smpr_init() is called to
initialize channel sampling time. The "st,min-sample-time-ns" property
is an optional property. If it is not defined, stm32_adc_smpr_init() is
currently skipped.
However stm32_adc_smpr_init() must always be called, to force a minimum
sampling time for the internal channels, as the minimum sampling time is
known. Make stm32_adc_smpr_init() call unconditional.

Fixes: 796e5d0b ("iio: adc: stm32-adc: use generic binding for sample-time")
Signed-off-by: NOlivier Moysan <olivier.moysan@foss.st.com>
Reviewed-by: NAndy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: NFabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20221012142205.13041-2-olivier.moysan@foss.st.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>