staging: comedi: adl_pci9118: DMA requires an interrupt

In order for DMA to work we also need an interrupt. Refactor the code
so that the DMA allocation is only done if the interrupt is available.

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>