stdio: Correct numbering logic in stdio_probe_device()

The current code assumes that the devices are ordered corresponding to
their alias value. But (for example) video1 can come before video0 in the
device tree.

Correct this, by always looking for device 0 first. After that we can fall
back to finding the first available device.

Signed-off-by: Simon Glass <sjg@chromium.org>