staging: comedi: addi_apci_16xx: board does not have digital inputs

The boards supported by this driver do not have digital inputs.
Remove the subdevice init for them.

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