staging: comedi: ni_atmio16d: cleanup atmio16d_ao_insn_write()

The comedi core validates the insn->chanspec and data values before
calling the (*insn_write) functions. The 'chan' will always be valid.

Tidy up this function up a bit and remove the unnecessary code.

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>