staging: comedi: addi_apci_3120: introduce apci3120_init_dma()

Introduce a helper function to handle the common code that writes the DMA
start address and number of acquisitions to the AMCC Add-on registers.

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>