staging: comedi: addi_apci_3120: introduce struct apci3120_dmabuf

For aesthetics, wrap the DMA buffer information in a struct.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>