mac80211: always allow calling ieee80211_connection_loss()

With multi-channel, there's a corner case where a driver
doesn't receive a beacon soon enough to be able to sync
its timers with the AP. In this case, the only recovery
(after trying again) is to disconnect from the AP. Allow
calling ieee80211_connection_loss() for such cases. To
make that possible, modify the work function to not rely
on the IEEE80211_HW_CONNECTION_MONITOR flag but use new
state kept in the interface instead.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>