staging: comedi: usbduxsigma: cleanup the (*cancel) functions

The (*cancel) functions can only be called by the comedi core it the
(*auto_attach) completed successfully. That function sets the comedi_device
'private' variable before initializing the callbacks so the sanity checks
are unnecessary. Remove them.

Also, rename the 'this_usbduxsub' local variable to 'devpriv' as this 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>