wpa_supplicant: Search BSS list from back to front

wpa_supplicant: Search BSS list from back to front

This fixes an issue seen in our wifi testbed, where we frequently
switch the SSID of the AP. wpa_supplicant's BSS list will have, for
example both <bssid, "Check11b"> (from the previous test) and <bssid,
"Check11g"> (from the current test) - note that the bssid is the same for
both. The (old) D-Bus API for fetching scan responses from wpa_supplicant
is fetch-by-bssid, so the caller (flimflam) incorrectly believes we're
associated with <bssid, "Check11b">.