staging: comedi: addi_apci_3xxx: only allocate needed subdevices

The number of subdevices needed by a given board supported by this
driver can cary from 2 (for the apci3500) to a maximum of 5. Currently
this driver always allocates 7 subdevices and sets the subdevice type
to COMEDI_SUBD_UNUSED for the ones that are not needed.

Calculate the actual number of needed subdevices from the boardinfo
and only allocate and setup the ones that are used.

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>