staging: comedi: adl_pci9118: factor out DMA alloc/free

For aesthetics, factor the DMA allocation/free code out of the attach
and detach functions.

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>