staging: comedi: usbduxfast: remove 'usb' from the private data

The usb_device pointer can be fetched when needed from the comedi_device
by using comedi_to_usb_interface() then interface_to_usbdev().

Do that instead and remove the pointer from the private data.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>