rustup: empty tuple -> ast::VariantData::Unit
rustup: empty tuple -> ast::VariantData::Unit

Fixes the following error which shows up with ioregs! generated code:

    error: empty tuple structs and enum variants are not
           allowed, use unit structs and enum variants instead

Signed-off-by: Paul Osborne <b6afffd6c1a00645fdd04b09edf63d034a949a39@gmail.com>

farcaller/zinc,phil-opp/zinc,posborne/zinc,posborne/zinc,richo/zinc,ntwyman/zinc,phil-opp/zinc,mcoffin/zinc,ntwyman/zinc,hackndev/zinc,ntwyman/zinc,farcaller/zinc,posborne/zinc,richo/zinc,mcoffin/zinc,mcoffin/zinc,troplin/zinc,mcoffin/zinc,richo/zinc,mcoffin/zinc,richo/zinc,farcaller/zinc,farcaller/zinc,richo/zinc,posborne/zinc,troplin/zinc,phil-opp/zinc,hackndev/zinc,ntwyman/zinc,troplin/zinc,troplin/zinc,ntwyman/zinc,posborne/zinc,farcaller/zinc,phil-opp/zinc,troplin/zinc,hackndev/zinc,hackndev/zinc,hackndev/zinc,phil-opp/zinc