staging: comedi: addi_apci_3120: provide analog output 'readback'

Use the comedi_subdevice 'readback' member and the core provided (*insn_read)
function to provide radback of the analog output channels.

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>