staging: comedi: addi_apci_1564: add Change-of-State interrupt subdevice and required functions

This board supports an interrupt that can be generated by an AND/OR
combination of 16 of the input channels.

Create a separate subdevice to handle this interrupt.

The apci1564_di_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 config instruction
INSN_CONFIG_DIGITAL_TRIG.

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

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