dm: serial: fix serial_post_probe()

The size of the name of a udevice is not limited.

When setting the fixed sized name field of a stdio device we must ensure
that the target string is NUL terminated to avoid buffer overflows.

Fixes: 57d92753 ("dm: Add a uclass for serial devices")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>