thunderbolt: Handle NULL boot ACL entries properly

If the boot ACL entry is already NULL we should not fill in the upper
two DWs with 0xfffffffff. Otherwise they are not shown as empty entries
when the sysfs attribute is read.

Fixes: 9aaa3b8b ("thunderbolt: Add support for preboot ACL")
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Yehezkel Bernat <yehezkelshb@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>