staging: comedi: ni_at_a2150: sample types are unsigned

Sample values in comedi are generally represented by unsigned values.
Change the type of the `dma_buffer` member of `struct a2150_private`
from `s16 *` to `uint16_t *`, and change the type of the `dpnt` variable
in `a2150_interrupt()` to `unsigned short` for consistency.

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