staging: comedi: addi_apci_3120: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Change the element type of `ul_DmaBufferVirtual[2]` in `struct
addi_private` from `short *` to `unsigned short *` for consistency.
Note that several ADDI-DATA drivers use this struct from
"addi_common.h", but only the "addi_apci_3120" driver uses this member.
Also change the type of the `dma_buffer` parameter of
`v_APCI3120_InterruptDmaMoveBlock16bit()` from `short *` to `unsigned
short *`.

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