mac80211: use DECLARE_EVENT_CLASS

For events that include only the local struct as
their parameter, we can use DECLARE_EVENT_CLASS
and save quite some binary size across segments
as well lines of code.

   text	   data	    bss	    dec	    hex	filename
 375745	  19296	    916	 395957	  60ab5	mac80211.ko.before
 367473	  17888	    916	 386277	  5e4e5	mac80211.ko.after
  -8272   -1408       0   -9680   -25d0 delta

Some more tracepoints with identical arguments
could be combined like this but for now this is
the one that benefits most.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>