firmware: implement muting of inputs

firmware/Include/main.h

@@ -55,6 +55,7 @@ void Error_Handler(void);
 
 void set_input(uint8_t channel);
 void set_attenuation(uint8_t left, uint8_t right);
+void set_mute(uint8_t mute);
 
 /* USER CODE BEGIN EFP */