mac80211: convert rssi_callback() to event_callback()

We will be able to add more events, such as MLME events and
others. The low level driver may be interested in knowing
about these events to dump firmware data upon failures, or
to change parameters in case connection attempts fail etc...

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>