staging: comedi: usbdux: tidy up usbduxsub_pwm_irq()

Rename the local variables to the comedi "norm".

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