AP_Compass: fixed last_update for secondary compass with primary failed

this ensures EKF and DCM will use a secondary compass if the primary
fails

libraries/AP_Compass/AP_Compass_PX4.cpp

@@ -129,9 +129,9 @@ bool AP_Compass_PX4::read(void)
         _count[i] = 0;
     }
 
-    last_update = _last_timestamp[0];
+    last_update = _last_timestamp[_get_primary()];
     
-    return _healthy[0];
+    return _healthy[_get_primary()];
 }
 
 void AP_Compass_PX4::accumulate(void)