of: Clean up of_update_property

After searching for the old property, bail out with -ENODEV
if it was not found.

It is unnecessary to check if oldprop is NULL before removing
its binary file; the check was already done before.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Rob Herring <robh@kernel.org>