AP_GPS: set GPS_SAVE_CFG default to save GPS config as required

libraries/AP_GPS/AP_GPS.cpp

@@ -153,7 +153,7 @@ const AP_Param::GroupInfo AP_GPS::var_info[] = {
     // @Description: Determines whether the configuration for this GPS should be written to non-volatile memory on the GPS. Currently working for UBlox 6 series and above.
     // @Values: 0:Do not save config,1:Save config,2:Save only when needed
     // @User: Advanced
-    AP_GROUPINFO("SAVE_CFG", 11, AP_GPS, _save_config, 0),
+    AP_GROUPINFO("SAVE_CFG", 11, AP_GPS, _save_config, 2),
 
     // @Param: GNSS_MODE2
     // @DisplayName: GNSS system configuration