Some AP equipment "in the wild" uses the same BSSID on multiple channels
(particularly "a" vs. "b/g").  This patch changes the key of sta_bss_list
to include both the BSSID and the channel so as to prevent a BSSID on
one channel from eclipsing the same BSSID on another channel.
Signed-off-by: John W. Linville <linville@tuxdriver.com>