staging: comedi: usbdux: tidy up usbduxsub_submit_pwm_urbs()

Pass the comedi_device pointer (the urb context) to this function
instead of the private data pointer.

Use a local variable for the urb pointer that is setup and submitted.

Remove the sanity check of the private data. This function can only
get called if the allocation was successful during the attach.

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>