staging: comedi: usbduxsigma: remove private data sanity checks

The comedi functions in this driver can only be called if the
(*auto_attach) successfull kzalloc'ed the private data and set
dev->private. The extra sanity checks are not needed. Remove them.

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>