staging: comedi: addi_apci_3120: enable AI async commands

The async command support should now work. Enable the hook up of the command
support functions in apci3120_auto_attach().

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>