mac80211: reset CQM history upon reconfiguration

The current behavior of notifying CQM events is inconsistent:
Upon first configuration there is a cqm event with the current
status according to threshold configured, regardless of signal
stability.
When there is reconfiguration no event is sent unless there is
a significant change to the signal level according to the new
configuration.

Since the current reconfiguration behavior might cause missing
CQM events in case the current signal did not change but is on
the other side of the new threshold, fix that by resetting the
stored signal level upon reconfiguration.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>