uhci-hcd: fix list access bug

When skipping to the last TD of an URB, go to the _last_ entry in the
list instead of the _first_ entry (as780).  This fixes Bugzilla #6747
and possibly others.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>