of: Constify safe_name() kobject arg

The kobject is not modified by safe_name() function, so make it const.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20241010-dt-const-v1-6-87a51f558425@kernel.org


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>