staging: comedi: addi_apci_2200: only allocate needed subdevices

The addi-data "common" code always allocated 7 subdevices. This
driver only requires 3. Change the allocation and remove the
unused subdevices.

Update the comments for the initialization of each subdevice.

Also, update the subdevice that is passed to addi_watchdog_cleanup()
to match the new subdevice number.

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>