UHCI: no dummy TDs for Iso QHs

Isochronous queues don't need a dummy TD because the Queue Header
isn't managed by the hardware.  This patch (as836) removes the
unnecessary dummy TDs.

The patch also fixes a long-standing typo in a comment (a "don't" was
missing -- potentially very confusing!).

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