most simple way to hang ahci(4): camcontrol tur on ada disks

Hi!

 After looking again at my `cdrecord -scanbus -V' log...
	http://lists.freebsd.org/pipermail/freebsd-current/2009-August/010660.html
...I remembered camcontrol(8) can send the test unit ready command as
well, so I booted into single user (to avoid having to fsck again)
and tried it - and lo and behold, the first `ls' afterwards hung again.
So its not something weird cdrecord/cdrdao do, a simple test unit ready
is enough to reproduce it, or at least here...

 (My guess is this has something to do with the fact that sata disks
are not atapi i.e. don't speak scsi commands by default so the
test unit ready would have to be either emulated or translated and
there's probably something missing or broken in that code?  Or maybe
scsi commands should just be rejected on non-atapi sata devices
for now?)

 HTH,
	Juergen