* Fix bug in wdc that would overflow ATAPI transfer length.
* Improve wdc probe code so that 'wdc' is probed in if present
even if there are no drives attached, and so that it works
properly even if the only device is an ATAPI slave.
* bus_space-ify.
* split the ISA attachment from the wdc driver, and remove
ISA dependencies from non-ISA files.
* claim that wd and wdc are now machine-independent (probably not
completely true, but mostly so; they at least work on arm32 and
i386).
* Various other minor fixups and cleanups, some of which were pointed
out by Kazuki Sakamoto.
