staging: comedi: amplc_pci224: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Change the type of the `ao_scan_vals` member of `struct pci224_private`
from `short *` to `unsigned short *` for consistency.  Also change the
type of the `array` variable in `pci224_ao_munge()` from `short *` to
`unsigned short *`.

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