staging: comedi: addi_apci_3xxx: stub in analog input command support

The analog input subdevice in this driver is broken. The attach of
this driver requests the irq and sets the dev->read_subdev to the
analog input subdevice but it does not setup the subdevice to support
commands.

Stub in the command support functions so we can fix the subdevice.

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>