staging: comedi: drivers (core): factor out async subdevice postconfig

Factor the setup of an async subdevice out of postconfig(). This allows
bringing the code back a couple indents and makes the postconfig a bit
clearer.

For aesthetic reasons, rename postconfig() to __comedi_device_postconfig().

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>