usb/isp1760: Clear TT buffer on interrupted low & full speed transfers

When a low or full speed urb in progress is unlinked (or some other error
occurs), the buffer in the transaction translator (part of the hub) might end
up in an inconsistent state. This can make all further low and full speed
transactions fail, unless the buffer is cleared.

The bug can be seen when running the usbtest unlink tests as "set altsetting
to 0 failed, -110", and gets fixed by this patch.

Signed-off-by: Arvid Brodin <arvid.brodin@enea.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>