staging: comedi: dt282x: IRQ and DMA are needed for async command support

This driver requires an IRQ and 2 DMA channels in order to execute async
commands.

Modify the (*attach) so that command support is only hooked up if
DMA is available. This allows removing the extra checks in the
command support 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>