AP_Compass: fix externalAHRS detection

'A = B >= C' kind. The expression is calculated as following: 'A = (B >= C)'

libraries/AP_Compass/AP_Compass.cpp

@@ -1206,7 +1206,8 @@ void Compass::_probe_external_i2c_compasses(void)
 void Compass::_detect_backends(void)
 {
 #if HAL_EXTERNAL_AHRS_ENABLED
-    if (int8_t serial_port = AP::externalAHRS().get_port() >= 0) {
+    const int8_t serial_port = AP::externalAHRS().get_port();
+    if (serial_port >= 0) {
         ADD_BACKEND(DRIVER_SERIAL, new AP_Compass_ExternalAHRS(serial_port));
     }
 #endif