staging: comedi: fl512: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Change the element type of the `ao_readback[]` member of `struct
fl512_private` from `short` to `unsigned short` for consistency.

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