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>

Author: rmurphy-arm

drivers/iommu/mtk_iommu_v1.c

@@ -509,14 +509,10 @@ static struct iommu_device *mtk_iommu_v1_probe_device(struct device *dev)
 
 static void mtk_iommu_v1_probe_finalize(struct device *dev)
 {
-	struct dma_iommu_mapping *mtk_mapping;
-	struct mtk_iommu_v1_data *data;
+	__maybe_unused struct mtk_iommu_v1_data *data = dev_iommu_priv_get(dev);
 	int err;
 
-	data        = dev_iommu_priv_get(dev);
-	mtk_mapping = data->mapping;
-
-	err = arm_iommu_attach_device(dev, mtk_mapping);
+	err = arm_iommu_attach_device(dev, data->mapping);
 	if (err)
 		dev_err(dev, "Can't create IOMMU mapping - DMA-OPS will not work\n");
 }