fix host issue when mounting an CDC device without IAD

Author: hathach

src/host/usbh.c

@@ -1522,7 +1522,7 @@ static bool _parse_configuration_descriptor(uint8_t dev_addr, tusb_desc_configur
 
 #if CFG_TUH_MIDI
     // MIDI has 2 interfaces (Audio Control v1 + MIDIStreaming) but does not have IAD
-    // manually increase the associated count
+    // manually force associated count = 2
     if (1                              == assoc_itf_count              &&
         TUSB_CLASS_AUDIO               == desc_itf->bInterfaceClass    &&
         AUDIO_SUBCLASS_CONTROL         == desc_itf->bInterfaceSubClass &&
@@ -1532,6 +1532,17 @@ static bool _parse_configuration_descriptor(uint8_t dev_addr, tusb_desc_configur
     }
 #endif
 
+#if CFG_TUH_CDC
+    // Some legacy CDC device does not use IAD but rather use device class as hint to combine 2 interfaces
+    // manually force associated count = 2
+    if (1                                        == assoc_itf_count              &&
+        TUSB_CLASS_CDC                           == desc_itf->bInterfaceClass    &&
+        CDC_COMM_SUBCLASS_ABSTRACT_CONTROL_MODEL == desc_itf->bInterfaceSubClass)
+    {
+      assoc_itf_count = 2;
+    }
+#endif
+
     uint16_t const drv_len = tu_desc_get_interface_total_len(desc_itf, assoc_itf_count, (uint16_t) (desc_end-p_desc));
     TU_ASSERT(drv_len >= sizeof(tusb_desc_interface_t));