eeepc-laptop: define rfkill notifier nodes only once

The rfkill notifier node names are used in three different places. As a
matter of style, it is better to store them somewhere and have the
compiler warn us about typos in the function arguments.

Signed-off-by: Frans Klaver <fransklaver@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>