iwlwifi: mvm: set AID to firmware only for associated stations

The firmware will soon actually look at the AID field, and
when it does that it'll try to ensure that the AID is never
changing. Due to the way the station is added, it may start
with an invalid AID before it's associated, so to ensure a
constant AID (once it becomes non-zero), track the station
state and set the AID only when the station is associated
and when it disassociates.

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