staging: comedi: addi_apci_16xx: only allocate needed subdevices

The addi-data "common" code always allocated 7 subdevices. This
driver only uses 1. Change the allocation and remove the unused
subdevices.

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>