staging: comedi: addi_apci_1500: remove unused subdevices

Only allocate space for the subdevicess used by the driver. Remove all
the COMEDI_SUBD_UNUSED subdevices.

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