from usbd_audio.h:205
typedef struct
{
uint8_t bLength;
uint8_t bDescriptorType;
uint8_t bDescriptorSubtype;
uint8_t bUnitID;
uint8_t bSourceID;
uint8_t bControlSize;
uint16_t bmaControls;
uint8_t iTerminal;
} __PACKED USBD_SpeakerFeatureDescTypeDef;