staging: comedi: pcl816: only hook up async command support if DMA is available

This driver only supports async commands if a DMA channel is available. Modify
the subdevice init so that the command support only gets hooked up if we can
do 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>