mac80211: pull mgmt frame rx into rx handler

Some code is duplicated between ibss, mesh and
managed mode regarding the queueing of management
frames. Since all modes now use a common skb
queue and a common work function, we can pull
the queueing code into the rx handler directly
and remove the duplicated length checks etc.

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