staging: comedi: usbdux: pass comedi_device pointer to usbdux_alloc_usb_buffers()

For aesthetic reasons, pass the comedi_device pointer to this function
instead of the private data pointer.

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>