mac80211: don't react to beacon loss if HW monitoring

mac80211: don't react to beacon loss if HW monitoring

If the HW is monitoring connection loss (as advertised
by IEEE80211_HW_CONNECTION_MONITOR) but not filtering
beacons (IEEE80211_VIF_BEACON_FILTER) then mac80211 will
still start the beacon loss timer and if a few beacons
are lost, e.g. due to scanning, drop the connection.

If the hardware doesn't advertise connection monitoring,
then it won't drop the connection right away but probe
the AP, which is intended, but due to the logic in the
timer when connection monitoring is done it assumes the
connection was actually lost.

Fix this problem by not starting the timer when the HW
does connection monitoring.

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