firmware-utils: mkchkmmg: fix chk header This is based on the work done by Michael Evans in https://bugs.openwrt.org/index.php?do=details&task_id=2009. I built this and successfully flashed the CHK onto an R8000 running the latest available Netgear firmware. Trying to flash the latest released image resulted in the same error reported in FS#2009. Signed-off-by: Thomas Willson <[email protected]>