Fix header offset calculation
coreboot stores the header position as a negative offset from the end of the flash device, which is why the high byte is 0xFF.
This fixes parsing the ADL boards which have 32 MiB WSON chips.
coreboot stores the header position as a negative offset from the end of the flash device, which is why the high byte is 0xFF.
This fixes parsing the ADL boards which have 32 MiB WSON chips.