Make USB MIDI device code allow a device with no Audio Control interface

src/device/usbd.c

@@ -920,7 +920,15 @@ static bool process_set_config(uint8_t rhport, uint8_t cfg_num)
           #endif
 
           #if CFG_TUD_MIDI
-          if ( driver->open == midid_open ) assoc_itf_count = 2;
+          if ( driver->open == midid_open )
+          {
+            // If there is a class-compliant Audio Control Class, then 2 interfaces
+            // Otherwise, only one
+            if (TUSB_CLASS_AUDIO               == desc_itf->bInterfaceClass    &&
+                        AUDIO_SUBCLASS_CONTROL         == desc_itf->bInterfaceSubClass &&
+                        AUDIO_FUNC_PROTOCOL_CODE_UNDEF == desc_itf->bInterfaceProtocol)
+             assoc_itf_count = 2;
+          }
           #endif
 
           #if CFG_TUD_BTH && CFG_TUD_BTH_ISO_ALT_COUNT