Make STA opmode change event available to upper layers
Add an event callback for EVENT_STATION_OPMODE_CHANGED to allow
user/application to get the notification whenever there is a change in a
station's HT/VHT op mode.
The new events:
STA-OPMODE-MAX-BW-CHANGED <addr> <20(no-HT)|20|40|80|80+80|160>
STA-OPMODE-SMPS-MODE-CHANGED <addr> <automatic|off|dynamic|static>
STA-OPMODE-N_SS-CHANGED <addr> <N_SS>