staging: comedi: aio_iiro_16: introduce aio_iiro_enable_irq()

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

Introduce a helper function to enable/disable the interrupt. Use the new
helper function to ensure that interrupts are initially disabled during
the driver (*attach).

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>