staging: comedi: ni_mio_common: tidy up the Digital I/O subdevice init

For aesthetics, add some whitespace to the Digital I/O subdevice init.

Only hook up the async command support if we have an irq.

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>