cmd_fdt: Actually fix fdt command in sandbox

Commit 90bac29a claims to fix this bug
that was introduced in commit a92fd657
but doesn't actually make the change that the commit message describes.

Actually fix the bug this time.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Simon Glass <sjg@chromium.org>