staging: comedi: pcl818: tidy up the analog output subdevice

For aesthetics, tidy up the analog output subdevice code.

Change the regsiter map defines to simplify the code.

Move the analog subdevice support functions out of the analog input
support functions. Tidy them up a bit during the move.

Reverse the logic of the subdevice init and add some whitespace.

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>