staging: comedi: addi_apci_1564: remove diagnostic interrupt support code

As per Ian, at this point in time it is not worth implementing an async
command interface for diagnostic interrupts for this board.  As this is
the case, this patch removes the code which supports such interrupts as it
is now unused.

This includes removing apci1564_do_read(), which was the insn_read
operation for the digital output subdevice, since all it was doing was
reading the current diagnostic interrupt type and returning it in 'data'.
This doesn't follow the comedi API and this operation can be emulated by
the comedi core anyway since the insn_bits operation follows the comedi
API.  So it is safe to simply remove this function.

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>