staging: comedi: adl_pci9118: introduce pci9118_amcc_dma_ena()

Introduce a helper function to program the AMCC master control/status
register to enable/disable dma.

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>