staging: comedi: usbduxsigma: tidy up usbduxsub_ai_IsocIrq()

Rename the local variables to follow the "norm" for comedi drivers.
The comedi_subdevice for this function is the dev->read_subdev that
was initialized during the (*auto_attach), use that instead of
directly accessing the comedi_device subdevices array.

Use dev->class_dev for any 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>