staging: comedi: das1800: sample types are unsigned

Sample values in comedi are generally represented by unsigned values.
Change the type of the `ao_update_bits` member of `struct
das1800_private` and the types of various local variables used to hold
sample values 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>