serial: Use next serial device if probing fails

Currently our serial device search chokes on the fact that the serial
probe function could fail. If it does, instead of searching for the next
usable serial device, it just quits.

This patch changes the fallback logic so that even when a serial device
was not probed correctly, we just try the next ones until we find one that
works.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>