staging: comedi: usbdux: tidy up usbdux_usb_probe()

Use a pointer to the struct usbdux_private data being configured
in the probe. Also use a pointer to the struct urb being setup in
the buffer allocations.
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>