staging: comedi: cb_pcidas: use subdevice readback for 'dac08_value'

Use the comedi_subdevice 'readback' member and the core provided (*insn_read)
to handle the readback of the write-only dac08 calib subdevice. Remove the
then unused 'dac08_value' member from the private data.

The dac08 calib subdevice only has one channel. For consistency in the driver,
modify the subdevice init so that a loop is used to initialize the channels and
readback values.

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>