staging: comedi: cb_pcidas64: fix ad8402_write_insn()

The comedi core expects the (*insn_write) functions to write 'insn->n'
values to the hardware and return the number of values written. Currently
this function only writes the first value. For this subdevice it only makes
sense to write the final data value.

Fix the function to work like the core expects. 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>