iommu/mediatek-v1: Fix a build warning for a unused variable 'data'

This patch fixes a build warning:
drivers/iommu/mtk_iommu_v1.c: In function 'mtk_iommu_release_device':
>> drivers/iommu/mtk_iommu_v1.c:467:25: warning: variable 'data' set but
>> not used [-Wunused-but-set-variable]
467 |  struct mtk_iommu_data *data;
|                         ^~~~

It's reported at:
https://lore.kernel.org/linux-iommu/202005191458.gY38V8bU%[email protected]/T/#u

Reported-by: kbuild test robot <[email protected]>
Signed-off-by: Yong Wu <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Joerg Roedel <[email protected]>

Author: YongWu-HF

drivers/iommu/mtk_iommu_v1.c

@@ -469,12 +469,10 @@ static void mtk_iommu_probe_finalize(struct device *dev)
 static void mtk_iommu_release_device(struct device *dev)
 {
 	struct iommu_fwspec *fwspec = dev_iommu_fwspec_get(dev);
-	struct mtk_iommu_data *data;
 
 	if (!fwspec || fwspec->ops != &mtk_iommu_ops)
 		return;
 
-	data = dev_iommu_priv_get(dev);
 	iommu_fwspec_free(dev);
 }