avc_audio_get_selector
int avc_audio_get_selector(struct fw_unit *unit, unsigned int subunit_id,
err = avc_audio_get_selector(bebob->unit, 0x00, 0x04,
err = avc_audio_get_selector(bebob->unit, 0, 9, &enable_ext);
err = avc_audio_get_selector(bebob->unit, 0, 8, &enable_word);
err = avc_audio_get_selector(bebob->unit, 0, 4, id);