cfg80211: move exported event functions into nl80211

This is the sort of thing gcc's LTO could do, but since
we don't have that yet we can also do it manually. The
advantage is reduced code, both source and binary, e.g.
on x86-64

   text	   data	    bss	    dec	    hex	filename
 442825	  56230	    776	 499831	  7a077	cfg80211.ko (before)
 441585	  56230	    776	 498591	  79b9f	cfg80211.ko (after)

a reduction of ~1k.

But in order to not complicate the code move only those
functions that are simple wrappers, not those that have
functionality of their own.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>