staging: comedi: pcmmio: only hook up async command support if irq is available

Refactor the board attach so that the async commands support is only
hooked up if the irq is actually 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>