usbstack: Revise usb string descriptor table to use enum values for indices

This makes it possible for macros of conditionally included string
descriptors to get a correct index no matter what other usb drivers
are enabled or disabled due to the nature behavior of enums.

Change-Id: I8ccebbd316605bed0f5d90b6b73fab4a333c02fa

firmware/export/usb_core.h

@@ -39,6 +39,14 @@
 
 extern int usb_max_pkt_size;
 
+enum {
+    USB_STRING_INDEX_LANGUAGE,
+    USB_STRING_INDEX_MANUFACTURER,
+    USB_STRING_INDEX_PRODUCT,
+    USB_STRING_INDEX_SERIAL,
+    USB_STRING_INDEX_MAX,
+};
+
 struct usb_class_driver;
 
 void usb_core_init(void);