AdvancedDAC: Make frequency const.

Author: iabdalkader

src/AdvancedDAC.cpp

@@ -182,7 +182,7 @@ int AdvancedDAC::stop()
     return 1;
 }
 
-int AdvancedDAC::frequency(uint32_t frequency)
+int AdvancedDAC::frequency(uint32_t const frequency)
 {
     if (descr && descr->pool) {
         // Reconfigure the trigger timer.

src/AdvancedDAC.h

@@ -49,7 +49,7 @@ class AdvancedDAC {
         void write(SampleBuffer dmabuf);
         int begin(uint32_t resolution, uint32_t frequency, size_t n_samples=0, size_t n_buffers=0);
         int stop();
-        int frequency(uint32_t frequency);
+        int frequency(uint32_t const frequency);
 };
 
 #endif /* ARDUINO_ADVANCED_DAC_H_ */