staging: comedi: usbduxsigma: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Use unsigned types consistently for handling comedi sample data and also
for the USB data buffers.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>