staging: comedi: cb_pcidas: fix dac08_write_insn()

As pointed out by Ian Abbott, the comedi INSN_WRITE instructions
are meant to iterate over, and write, all the data[] passed from
the comedi core. Modify dac08_write_insn() to work as intended.

Since doc08_write_insn() now returns the proper response to the
core, make the dac08_write() helper return void.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reported-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>