apm/controller: Make GetCurrentPerformanceMode() a const member function

This doesn't modify instance state, so it can be made const qualified.

src/core/hle/service/apm/controller.cpp

@@ -48,7 +48,7 @@ void Controller::SetFromCpuBoostMode(CpuBoostMode mode) {
                                 BOOST_MODE_TO_CONFIG_MAP.at(static_cast<u32>(mode)));
 }
 
-PerformanceMode Controller::GetCurrentPerformanceMode() {
+PerformanceMode Controller::GetCurrentPerformanceMode() const {
     return Settings::values.use_docked_mode ? PerformanceMode::Docked : PerformanceMode::Handheld;
 }
 

src/core/hle/service/apm/controller.h

@@ -56,7 +56,7 @@ public:
     void SetPerformanceConfiguration(PerformanceMode mode, PerformanceConfiguration config);
     void SetFromCpuBoostMode(CpuBoostMode mode);
 
-    PerformanceMode GetCurrentPerformanceMode();
+    PerformanceMode GetCurrentPerformanceMode() const;
     PerformanceConfiguration GetCurrentPerformanceConfiguration(PerformanceMode mode);
 
 private: