Set the data toggle correctly, and use EHCI_QTD_DTC.  This fixes
problems with my ALi-based drive enclosure (it works now, rather
than failing to attach).  Also seems to work with a GL811-based
enclosure and an ASUS enclosure with a CD-RW, on both Intel and
NEC controllers.

Note: The ALi enclosure is currently very SLOW, due to some issue
with taking too long to notice that the QTD is complete.  This
requires more investigation.

Failure to properly mask off UE_DIR_IN from the endpoint address
was causing OHCI_ED_FORMAT_ISO and EHCI_QH_HRECL to get set
spuriously, causing rather interesting lossage.

Suddenly I get MUCH better performance with ehci...

Fix a stupid bug in ehci_check_intr() that caused use to try to
complete a transaction that was still running.  Now ehci can
handle multiple devices being active at once.

As the ehci_idone() now uses the variable `epipe'
unconditionally, always declare it (in other words, make this
file compile w/o EHCI_DEBUG).

Remove comment about the data toggle being borked.

Update comment.

Adjust a couple of comments to make it clear WTF is going on.

Fix an error in a debug printf().

Further cleanup of toggle handling.  Now that we use EHCI_QH_DTC,
we don't need to fiddle with the TOGGLE bit in the overlay
descriptor, so minimize how much we fuss with it.