staging: comedi: addi_apci_2032: always initialize interrupt subdevice

Initialize the special interrupt subdevice as a digital input subdevice
even if the interrupt handler cannot be registered.  It's `insn_bits`
handler will still read the interrupt status register.  This hardware
status bits in this register might be valid even if they haven't been
enabled in the interrupt control register, but this needs to be checked.
In any case, initializing the subdevice as a digital input subdevice is
harmless.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>