staging: comedi: addi_apci_1032: hook-up the interrupt subdevice

The board supported by this driver can generate an interrupt based
on the state of input channels 0-15.

The apci1032_intr_insn_config() function is used to configure which
inputs are used to generate the interrupt. Currently this function
is broken since it does not follow the comedi API for insn_config
functions. Fix this function by implementing the, currently unused,
config instruction INSN_CONFIG_DIGITAL_TRIG.

Add the remaining subdevice operations necessary for the interrupt
subdevice to support async commands.

Fix the subdevice initialization so that if the interrupt is not
available the subdevice is set as COMEDI_SUBD_UNUSED.

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