bootdev: avoid infinite probe loop

Sometimes, when only one bootdev is available, and it fails to probe, we
end up in an infinite loop calling probe() on the same device over and
over. With only debug level log output.

Break the loop if we fail to probe the same device twice in a row, and
promote the probe failure message to log_warning().

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Dragan Simic <dsimic@manjaro.org>