USB: add a private-data pointer to struct usb_tt

For improved scheduling of transfers through a Transaction Translator,
ehci-hcd will need to store a bunch of information associated with the
FS/LS bus on the downstream side of the TT.  This patch adds a pointer
for such HCD-private data to the usb_tt structure.

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