iommu/mediatek-v1: Tidy up probe_finalize

iommu/mediatek-v1: Tidy up probe_finalize

Krzysztof points out that although the driver now supports COMPILE_TEST
for other architectures, it does not build cleanly with W=1 where the
stubbed-out ARM API can lead to an unused variable warning.

Since this is effectively the correct intent of the code in such cases,
mark it as __maybe_unused, tidying up some cruft in the process.

Reported-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Yong Wu <yong.wu@mediatek.com>
Link: https://lore.kernel.org/r/7c78149504900bc6c98a9c48f4418934b72d89ac.1751036478.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>