staging: comedi: pcmmio: tidy up the ao subdevice init

Add some whitespace to the analog output subdevice init and reorder the
init to follow the "norm" in comedi drivers.

Remove the init of len_chanlist. This member is only used for subdevices
the support async commands. The core will default the value correctly.

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>