staging: comedi: ni_labpc_common: use subdevice readback for analog outputs
Use the comedi_subdevice 'readback' member and the core provided (*insn_read)
to handle the readback of the analog output subdevice. Remove the then unused
'ao_value' member from the private data.
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>