- Factor out front part of ieee80211_set_shortpreamble() into

- Factor out front part of ieee80211_set_shortpreamble() into
  ieee80211_update_shpreamble().  So that the former only turns
  on/off short preamble and informs driver about the changes,
  while the latter makes the decision whether short preamble
  should be turned on or off.
- Replace rest of the places where short preamble is turned on/off
  manually with ieee80211_set_shortpreamble()
- For associated STAs, update short preamble state only when related
  fields of ERP information or capability information in received
  beacons are changed