mac80211: Fix reassociation by not clearing previous BSSID

We must not clear the previous BSSID when roaming to another AP within
the same ESS for reassociation to be used properly. It is fine to
clear this when the SSID changes, so let's move the code into
ieee80211_sta_set_ssid().

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>