[usb] Use non-zero language ID to retrieve strings

We currently use a zero language ID to retrieve strings such as the
ECM/NCM MAC address.  This works on most hardware devices, but is
known to fail on some software emulated CDC-NCM devices.

Fix by using the first supported language ID, falling back to English
(0x0409) if any error occurs when fetching the list of supported
languages.  This matches the behaviour of the Linux kernel.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/usb.h

@@ -223,6 +223,9 @@ struct usb_string_descriptor {
 /** A USB string descriptor */
 #define USB_STRING_DESCRIPTOR 3
 
+/** Language ID for English */
+#define USB_LANG_ENGLISH 0x0409
+
 /** A USB interface descriptor */
 struct usb_interface_descriptor {
 	/** Descriptor header */
@@ -728,6 +731,9 @@ struct usb_device {
 	struct usb_endpoint control;
 	/** Completed control transfers */
 	struct list_head complete;
+
+	/** Default language ID (if known) */
+	unsigned int language;
 };
 
 /** USB device host controller operations */