staging: comedi: addi_apci_1500: remove unnecessary subdevice initialization

The subdevices that don't initialize any of the callbacks don't really
exist. Remove the unnecessary initialzation and just set the type to
COMEDI_SUBD_UNUSED.

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>