Add a way to return the RSSI value (#934)

See https://forums.raspberrypi.com/viewtopic.php?t=341774

Author: mecparts

libraries/WiFi/src/WiFiClass.cpp

@@ -323,8 +323,13 @@ uint8_t* WiFiClass::BSSID(uint8_t* bssid) {
     return: signed value
 */
 int32_t WiFiClass::RSSI() {
-    // TODO - driver does not return this?!
-    return 0;
+#ifndef CYW43_IOCTL_GET_RSSI
+#define CYW43_IOCTL_GET_RSSI 0xFE
+#endif
+
+    int32_t rssi;
+    cyw43_ioctl(&cyw43_state, CYW43_IOCTL_GET_RSSI, sizeof rssi, (uint8_t *)&rssi, CYW43_ITF_STA);
+    return rssi;
 }
 
 /*