staging: comedi: addi_apci_2032: fix interrupt support

This board supports two interrupt sources:

VCC : detects when the external supply voltage drops below 5V
CC  : over temperature diagnostic

Currently the interrupt support is tied into the digital output
subdevice. It's also broken since it does not follow the comedi
API.

Create a new digital input subdevice to handle the interrupts.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>