staging: comedi: addi_apci_1564: add subdevice to check diagnostic status

This board provides VCC/CC diagnostic information, and it also supports
diagnostic interrupts.  However, as per Ian, these interrupts aren't very
useful and it is enough to simply provide an interface for accessing the
diagnostic status on-demand.  This patch adds a 2-channel digital input
subdevice with an insn_bits handler to access this information.

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>