fix PUSB_getInterface return value

Author: facchinm

Diff for: cores/arduino/USB/PluggableUSB.cpp

@@ -40,7 +40,7 @@ int PUSB_GetInterface(uint8_t* interfaceNum)
 	int ret = 0;
 	PUSBListNode* node = rootNode;
 	for (uint8_t i=0; i<modules_count; i++) {
-		ret = node->cb->getInterface(interfaceNum);
+		ret += node->cb->getInterface(interfaceNum);
 		node = node->next;
 	}
 	return ret;

Diff for: cores/arduino/USB/USBCore.cpp

@@ -156,6 +156,8 @@ uint32_t USBDeviceClass::sendConfiguration(uint32_t maxlen)
 		return true;
 	}
 
+	total = 0;
+
 	packMessages(true);
 	sendControl(&config, sizeof(ConfigDescriptor));
 	SendInterfaces(&total);