staging: comedi: comedi_test: ao subdevice does not support async commands

staging: comedi: comedi_test: ao subdevice does not support async commands

The comedi_subdevices array is kzalloc()'d so the async command callbacks do
not need to be set to NULL when the analog output subdevice is initialized.

Also, remove the initialzation of the subdevice 'len_chanlist' since this
value is only used by the async commands. The core will default it to 1.

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>