staging: comedi: pcl726: tidy up pcl726_ao_insn_read()

Tidy up this function to follow the normal form for analog output
read back functions.

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>