only run system() command when vol has changed (#1148)

* only run system command when vol has changed

* fix indent

selfdrive/ui/sound.cc

@@ -13,12 +13,17 @@ extern "C"{
 #include "slplay.h"
 }
 
+int last_volume = 0;
+
 void set_volume(int volume) {
+  if (last_volume != volume) {
     char volume_change_cmd[64];
     sprintf(volume_change_cmd, "service call audio 3 i32 3 i32 %d i32 1 &", volume);
 
     // 5 second timeout at 60fps
     int volume_changed = system(volume_change_cmd);
+    last_volume = volume;
+  }
 }