mac80211: implement sta_add/sta_remove in sta_state

Instead of maintaining separate sta_add/sta_remove
callsites, implement it in sta_state when the driver
has no sta_state implementation.

The only behavioural change this should cause is in
secure mesh mode: with this the station entries will
only be created after the stations are set to AUTH.
Given which drivers support mesh, this seems to not
be a problem.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>