staging: comedi: usbduxsigma: tidy up usbduxsigma_attach_common()

Rename the local variable used for the comedi_device private data.

Remove all the unnecessary comments and add some whitespace to the
subdevice init.

Use dev->class_dev for the dev_{level} messages.

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>