cfg80211: add some missing fall through annotations

There are talks about enabling -Wimplicit-fallthrough warnings in the
mainline and it is already enabled in linux-next.  Add all the
missing annotations to prevent warnings when this happens.

And in one case, remove the extra text from the annotation so that the
compiler recognizes it.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>