iio: adc: stm32-adc: split channel init into several routines

Split stm32_adc_chan_of_init channel initialization function into
several routines to increase readability and prepare channel
generic binding handling.

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/20211014131228.4692-4-olivier.moysan@foss.st.com


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