staging: comedi: quatech_daqp_cs: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Change `daqp_interrupt()` to use unsigned sample values for consistency.

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