[efi] Fix reporting of USB supported languages array

The length as returned by UsbGetSupportedLanguages() should not
include the length of the descriptor header itself.

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

src/include/ipxe/efi/efi_usb.h

@@ -24,7 +24,9 @@ struct efi_usb_device {
 	/** Configuration descriptor */
 	struct usb_configuration_descriptor *config;
 	/** Supported languages */
-	struct usb_descriptor_header *languages;
+	uint16_t *lang;
+	/** Length of supported languages */
+	size_t lang_len;
 	/** List of interfaces */
 	struct list_head interfaces;
 };