staging: comedi: addi_apci_3120: move DMA init code to apci3120_init_dma()

Move the common code used to initialize DMA to apci3120_init_dma().

This follows the programming procedure described in the APCI-3120
documentation.

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>