staging: comedi: ni_mio_common: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Change the element type of the `ai_fifo_buffer[]` element of `struct
ni_board_struct` and the types of various local variables 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>