staging: comedi: usbduxsigma: cleanup the unlink urb functions

These functions always return success. Change them to just return void.

The 'usbduxsub_tmp', which is actually the comedi_device private data, is
already validated by the callers. Remove the unnecessary validation and
rename the 'usbduxsub_tmp' parameter to 'devpriv' which is more common in
comedi drivers.

Remove the unnecessary comments.

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>