staging: comedi: aio_aio12_8: add ai and ao only card versions

Add the analog input and analog output only versions of the card
to the boardinfo. These cards all share the same i/o map.

Cleanup the attach so that the unsupported subdevices are not
initialized.

All of these boards also have an 8254 counter/timer. The 8254
device is not currently supported but stub in the subdevice
information so it can easily be added.

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>