iio: adc: palmas: move eventX_enable into palmas_adc_event

It just makes more sense to have all information regarding adc events in
one place.

Signed-off-by: Patrik Dahlström <risca@dalakolonin.se>
Link: https://lore.kernel.org/r/20230408114825.824505-7-risca@dalakolonin.se


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