Make ADC check_read_discard private

[rnd-ash]

Summary

Makes Adc::check_read_discard private in the ADC Module, as it shouldn't be exposed to the end user, and only the crate uses it internally

Checklist

• All new or modified code is well documented, especially public items
• No new warnings or clippy suggestions have been introduced - CI will deny clippy warnings by default! You may #[allow] certain lints where reasonable, but ideally justify those with a short comment.