mac80211: make deauth/disassoc sequence more natural

The association sequence looks (roughly) like
this now:
 * set BSSID
 * set station to EXIST state
 * send auth
 * set station to AUTH state
 * send assoc
 * set station to ASSOC state
 * set BSS info to associated

In contrast, the deauth/disassoc sequence is
the other way around:
 * clear BSSID/BSS info state
 * remove station
 * send deauth/disassoc
(in some cases the last two steps are reversed.)

This patch encodes the entire sequence in the
ieee80211_set_disassoc() function and changes
it to be like this, for good measure with an
explicit flush:
 * send deauth/disassoc
 * flush
 * remove station
 * clear BSSID/BSS info state

At least iwlwifi gets confused with the other
sequence in P2P mode and complains that it
wasn't able to flush the queues.

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