938: Do not use cfg_if for embedded-hal-async feature gates. r=Dirbaio a=Dirbaio
Old code used `cfg_if!` because rustc still parses code inside disabled cfg's, and Rust stable at that time couldn't parse the new GAT where-clause location. This is not the case anymore.
bors r+
Co-authored-by: Dario Nieuwenhuis <dirbaio@dirbaio.net>