lib: Suppress E when writing error-string output

When CONFIG_ERRNO_STR is not enabled this shows a spurious 'E' from the
format string. Fix this.

Fixes: 7f331941 ("lib: Support printing an error string")
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>