nl80211: Update assoc_freq and bss->freq based on real association info

nl80211: Update assoc_freq and bss->freq based on real association info

Move event.assoc_info.freq selection to be after the
nl80211_get_assoc_ssid() call so that the current cfg80211 information
on the operating channel can be used should anything unexpected have
happened between the association request and completion of association.
Furthermore, update bss->freq based on assoc_freq to make that
information a bit more useful for station mode. It was already updated
after channel switches during association, but not at the beginning of
association.

Signed-off-by: Jouni Malinen <j@w1.fi>