iio: adc: stm32-adc: add debug info if dt uses legacy channel config

Since nearly all stm32 dt's are using the legacy adc channel config,
we should warn users about using it.

Signed-off-by: Sean Nyekjaer <sean@geanix.com>
Link: https://lore.kernel.org/r/20230421083858.2613289-1-sean@geanix.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>