staging: comedi: aio_iiro_16: add command support for change of state detection

This board supports interrupts on change of state of the digital inputs.

Add the necessary subdevice support and interrupt handler to allow async
commands to detect the change of state.

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>