staging: comedi: comedi_test: make samples unsigned

Represent the fake samples as unsigned shorts instead of signed shorts,
as this is the usual Comedi convention.  There is no change to the
actual binary representation, although the `cfc_write_to_buffer()` call
currently expects a signed short for some bizarre reason.

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