mac80211: restructure disassoc/deauth flows

This patch restructure the flow of disassociation and deauthentication
flows to be consistent under all circumstances.
It ensures that BA session is treated down before deauthentication or disassociation,
adds the removal of the obsolete sta form station table and fixes a related bug (sta_info_destroy
without sta_info_unlink) in ieee80211_associated()
and reduce some code duplication

Signed-off-by: Ron Rindjunsky <ron.rindjunsky@intel.com>
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>