iio:adc:stm32*: Use pm[_sleep]_ptr() etc to avoid need to make pm __maybe_unused

The combinations of either
* pm_sleep_ptr() and DEFINE_SIMPLE_DEV_PM_OPS()
* pm_ptr() and RUNTIME_PM_OPS()/SYSTEM_SLEEP_PM_OPS
Make sure the functions are always visible to the compiler and removed by
it rather than requring #ifdef magic.

This removes the need to mark the functions as __maybe_unused and saves
additional space with some build options as the dev_pm_ops structure
itself can be dropped automatically if CONFIG_PM is not enabled.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Olivier Moysan <olivier.moysan@st.com>
Cc: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20220130193147.279148-51-jic23@kernel.org