staging: comedi: das1800: tidy up das1800_ao_insn_write()

The (*insn_write) functions are supposed to write insn->n values to the
specified channel. Tidy up this function and make it work like the core
expects.

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>