staging: comedi: usbduxsigma: tidy up usbduxsub_pwm_irq()

Rename the local variables to follow the "norm" for comedi drivers.
The comedi_subdevice is not used in this function. Remove that local
variable.

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

Remove the extra check of 'pwm_cmd_running' before submitting the urb.
This flag was previously checked.

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>