AP_AHRS: reuse ins object for multiple calls, simplify delta_t variable

libraries/AP_AHRS/AP_AHRS_DCM.cpp

@@ -69,23 +69,21 @@ AP_AHRS_DCM::update(bool skip_ins_update)
 {
     // support locked access functions to AHRS data
     WITH_SEMAPHORE(_rsem);
-    
-    float delta_t;
 
     if (_last_startup_ms == 0) {
         _last_startup_ms = AP_HAL::millis();
         load_watchdog_home();
     }
 
+    AP_InertialSensor &_ins = AP::ins();
+
     if (!skip_ins_update) {
         // tell the IMU to grab some data
-        AP::ins().update();
+        _ins.update();
     }
 
-    const AP_InertialSensor &_ins = AP::ins();
-
     // ask the IMU how much time this sensor reading represents
-    delta_t = _ins.get_delta_time();
+    const float delta_t = _ins.get_delta_time();
 
     // if the update call took more than 0.2 seconds then discard it,
     // otherwise we may move too far. This happens when arming motors