staging: comedi: usbdux: rename dev->private local variables

For aesthetics, consistently use the same name. 'devpriv', for the
local variable used to reference the private data, 'dev->private'.

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