staging: comedi: amplc_pci230: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Replace all uses of `short` to handle sample values in the
"amplc_pci230" driver with `unsigned short` for consistency.

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