image-host: fix 'unknown error' error message

Fix error message like this:
Can't add verification data for node 'fdt-1' (<unknown error>)

We get unknown error because we decode error as fdt error
but actually it is system error.

Signed-off-by: Alexander Kochetkov <al.kochet@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
1 file changed