net/ncsi: use proper "mellanox" DT vendor prefix

"mlx" Devicetree vendor prefix is not documented and instead "mellanox"
should be used.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220622115416.7400-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>