fdt: Tidy up error handling in image_setup_libfdt()

The message about needing to reset should be printed no matter what error
is printed. Also, an error should always be printed.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Tom Rini <trini@ti.com>