staging: comedi: usbduxfast: rename usbduxfastsub_s variables

For aesthetic reasons, globally rename the variables used for the
struct usbduxfastsub_s * in this driver to 'devpriv'. This variable
pointes to the comedi_device private data.

Also, rename the struct to 'usbduxfast_private' to make its use
a bit clearer.
Signed-off-by: NH Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>