staging: comedi: cb_pcidas: tidy up dac08_write_insn()

For consistency with the other calib subdevices, refactor this (*insn_write)
function so that it only writes the final data value to the hardware.,

For aesthetics, rename the function so it has namespace associated with the
driver.

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>