staging: comedi: fl512: tidy up fl512_ao_insn_read()

For aesthetics, tidy up this function to match the style used in most
of the comedi drivers for analog output (*insn_read) 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>