staging: comedi: pcl711: tidy up pcl711_ao_insn()

For aesthetics, rename this function to help with greps.

Introduce a helper function to write the values to the analog
output channel registers. Modify the defines for the D/A registers
to avoid the ? : tests based on the chan number.

Only the final value written needs to be saved for read back.

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>