wifi: fix scanned signal strength calculation for WEXT-based drivers

The new wpa_supplicant D-Bus interface only passes back the 'level'
of the scanned BSS, which with nl80211 drivers is almost always dBm,
which NM handled fine.  But WEXT-based drivers (ipw2x00, other older
ones, and some vendor drivers) use a mix of values for the 'level'
parameter, including the old WEXT 8-bit signed-value-in-unsigned-int
scheme.  Handle that.

Alternatively, we could have the supplicant expose the 'flags' value
from its internal BSS list over the bus.

src/nm-wifi-ap-utils.h

@@ -39,5 +39,7 @@ gboolean nm_ap_utils_complete_connection (const GByteArray *ssid,
                                           gboolean lock_bssid,
                                           GError **error);
 
+guint32 nm_ap_utils_level_to_quality (gint val);
+
 #endif  /* NM_WIFI_AP_UTILS_H */