staging: comedi: usbdux: rename struct usbduxsub

This struct is the comedi_device private data. For aesthetic reasons,
rename it to usbdux_private.

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>