Fix a device reference leak in st{read,write}() by making them work like {cd,sd}{read,write}().
Fix a device reference leak in st{read,write}() by making them work
like {cd,sd}{read,write}().

ok krw@, dlg@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars