audio: Add two channel feature unit descriptor

Feature unit descriptor size depends on number of channels.
So far only single channel feature unit was present.
This change adds feature unit descriptor for two channels.

src/device/usbd.h

@@ -350,6 +350,11 @@ TU_ATTR_WEAK bool tud_vendor_control_complete_cb(uint8_t rhport, tusb_control_re
 #define TUD_AUDIO_DESC_FEATURE_UNIT_ONE_CHANNEL(_unitid, _srcid, _ctrlch0master, _ctrlch1, _stridx) \
   TUD_AUDIO_DESC_FEATURE_UNIT_ONE_CHANNEL_LEN, TUSB_DESC_CS_INTERFACE, AUDIO_CS_AC_INTERFACE_FEATURE_UNIT, _unitid, _srcid, U32_TO_U8S_LE(_ctrlch0master), U32_TO_U8S_LE(_ctrlch1), _stridx
 
+// 2 - Channels
+#define TUD_AUDIO_DESC_FEATURE_UNIT_TWO_CHANNEL_LEN (6+(2+1)*4)
+#define TUD_AUDIO_DESC_FEATURE_UNIT_TWO_CHANNEL(_unitid, _srcid, _ctrlch0master, _ctrlch1, _ctrlch2, _stridx) \
+		TUD_AUDIO_DESC_FEATURE_UNIT_TWO_CHANNEL_LEN, TUSB_DESC_CS_INTERFACE, AUDIO_CS_AC_INTERFACE_FEATURE_UNIT, _unitid, _srcid, U32_TO_U8S_LE(_ctrlch0master), U32_TO_U8S_LE(_ctrlch1), U32_TO_U8S_LE(_ctrlch2), _stridx
+
 // For more channels, add definitions here
 
 /* Standard AS Interface Descriptor(4.9.1) */