mac80211: split managed/ibss code a little more

It appears that you can completely mess up mac80211 in IBSS
mode by sending it a disassoc or deauth: it'll stop queues
and do a lot more but not ever do anything again. Fix this
by not handling all those frames in IBSS mode,

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