staging: comedi: usbduxsigma: Fixed wrong range for the analogue channel.

It's actually +/-2.65V/2 and not +/-2.65V.

Signed-off-by: Bernd Porr <berndporr@f2s.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>