staging: comedi: dt3000: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Change the type of the `data` variable in `dt3k_ai_empty_fifo()` 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>