USB: cxacru: document how to interact with the flash memory

These commands were found by accident... fortunately
it still works even if the flash memory is erased,
despite having no USB device IDs.

Some example sysfs code for raw command access:
  http://simon.arlott.org/pub/cxacru/raw.c

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/atm/cxacru.c

@@ -105,6 +105,26 @@ enum cxacru_cm_request {
 	CM_REQUEST_MAX,
 };
 
+/* commands for interaction with the flash memory
+ *
+ * read:  response is the contents of the first 60 bytes of flash memory
+ * write: request contains the 60 bytes of data to write to flash memory
+ *        response is the contents of the first 60 bytes of flash memory
+ *
+ * layout: PP PP VV VV  MM MM MM MM  MM MM ?? ??  SS SS SS SS  SS SS SS SS
+ *         SS SS SS SS  SS SS SS SS  00 00 00 00  00 00 00 00  00 00 00 00
+ *         00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00
+ *
+ *   P: le16  USB Product ID
+ *   V: le16  USB Vendor ID
+ *   M: be48  MAC Address
+ *   S: le16  ASCII Serial Number
+ */
+enum cxacru_cm_flash {
+	CM_FLASH_READ = 0xa1,
+	CM_FLASH_WRITE = 0xa2
+};
+
 /* reply codes to the commands above */
 enum cxacru_cm_status {
 	CM_STATUS_UNDEFINED,