Some AP equipment "in the wild" services multiple SSIDs using the
same BSSID.  This patch changes the key of sta_bss_list to include
the SSID as well as the BSSID and the channel so as to prevent one
SSID from eclipsing another SSID with the same BSSID.
Signed-off-by: John W. Linville <linville@tuxdriver.com>