mac80211: better fix for conn_mon_timer running after disassociate

Is still possible to schedule conn_mon_timer after disassociate from
ieee80211_sta_tx_notify() and ieee80211_offchannel_ps_disable().

Move disassociate check to ieee80211_sta_reset_conn_monitor() to cover
all these cases, and add unlikely since in most the time we call
ieee80211_sta_reset_conn_monitor() when associated.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>