Don't call wdcintr() for channels that aren't properly initialized or don't have any drives on them.  Fixes a panic seen on sparc64 machines with that pesky Acer Labs chipset.
Don't call wdcintr() for channels that aren't properly initialized or don't
have any drives on them.  Fixes a panic seen on sparc64 machines with that
pesky Acer Labs chipset.

ok deraadt@, krw@, henning@

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