staging: comedi: fl512: tidy up fl512_ao_insn_write()

Tidy up this function. Only save the last value written for readback.

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>