AP_Stats: use set_and_save_ifchanged()

if the board has no GPS then the values usually don't change

libraries/AP_Stats/AP_Stats.cpp

@@ -60,8 +60,8 @@ void AP_Stats::init()
 
 void AP_Stats::flush()
 {
-    params.flttime.set_and_save(flttime);
-    params.runtime.set_and_save(runtime);
+    params.flttime.set_and_save_ifchanged(flttime);
+    params.runtime.set_and_save_ifchanged(runtime);
 }
 
 void AP_Stats::update_flighttime()
@@ -91,12 +91,11 @@ void AP_Stats::update()
         flush();
         last_flush_ms = now_ms;
     }
-
     const uint32_t params_reset = params.reset;
     if (params_reset != reset || params_reset == 0) {
-        params.bootcount.set_and_save(params_reset == 0 ? 1 : 0);
-        params.flttime.set_and_save(0);
-        params.runtime.set_and_save(0);
+        params.bootcount.set_and_save_ifchanged(params_reset == 0 ? 1 : 0);
+        params.flttime.set_and_save_ifchanged(0);
+        params.runtime.set_and_save_ifchanged(0);
         uint32_t system_clock = 0; // in seconds
         uint64_t rtc_clock_us;
         if (AP::rtc().get_utc_usec(rtc_clock_us)) {
@@ -105,7 +104,7 @@ void AP_Stats::update()
             // time base to Jan 1st 2016:
             system_clock -= 1451606400;
         }
-        params.reset.set_and_save(system_clock);
+        params.reset.set_and_save_ifchanged(system_clock);
         copy_variables_from_parameters();
     }