Verify that the selected BSS has a better signal level before roaming

Verify that the selected BSS has a better signal level before roaming

This prevents situations like the following where we roam to a
lesser quality BSS just because the signal level delta is over our
threshold.

wlan0: Considering within-ESS reassociation
wlan0: Current BSS: 00:24:6c:74:0a:40 level=-51
wlan0: Selected BSS: 00:24:6c:74:0a:e0 level=-64
wlan0: Request association: reassociate: 0  selected: 00:24:6c:74:0a:e0
    bssid: 00:24:6c:74:0a:40  pending: 00:00:00:00:00:00  wpa_state: COMPLETED

Signed-hostap: Robert Shade <robert.shade@gmail.com>