staging: comedi: usbdux: tidy up usbduxsub_ai_isoc_irq()

Rename the local variables to the comedi "norm".

The comedi_subdevice in this function is actually the dev->read_subdev
that was initialized in the attach. Use that instead of accessing the
dev->subdevices array directly.

Use dev->class_dev as the device for all dev_printk() messages.

Instead of using the 'comedidev' back pointer in the private data, use
the comedi_device 'dev' that we already have.

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>