mac80211: use common work function

Even with the previous patch, IBSS, managed
and mesh modes all attach their own work
function to the shared work struct, which
means some duplicated code. Change that to
only have a frame processing function and a
further work function for each of them and
share some common code.

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