Make mac80211_hwsim_sta_rc_update() iterate over all the STA links.

This is somewhat temporary, we really should add the link to the
API, but then hwsim still calls it internally and would need this.
Signed-off-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>