staging: comedi: adl_pci9118: enable DMA in common code path

The pci9118_ai_docmd_dma() function enables the DMA bit in the ai control
register for all acquisition modes. For aesthetics, move the enable of
this bit into the (*do_cmd).

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>