staging: comedi: das16m1: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Change the element type of the `ai_buffer[]` member of `struct
das16m1_private_struct` and the types used by `munge_sample()` and
`munge_sample_array()` 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>