mac80211: remove disassociation code from ieee80211_set_associated

This patch moves disassociation code from ieee80211_set_associated
to ieee80211_set_disassoc. To reduce code duplication, it introduces
the ieee80211_sta_send_apinfo function. Additionally, it fixes a lapse
where BSS_CHANGED_HT wasn't set when notifying the driver of changes
due to disassociation.

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