staging: comedi: usbduxsigma: fixing rounding error in the range struct

The AD converter on the usbsigma turns vref (2.5V/2) into the
binary value 0x780000. However the full scale range goes up to
0x800000 so that vref/2 needs to be upscaled by 0x800000/0x780000
which is 1.06bar. In the driver it's been 1.06 without the infinite
6666. I've put the fraction in the code so that the compiler can
calculate the most precise scaling value by itself.

Signed-off-by: Bernd Porr <mail@berndporr.me.uk>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>