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

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%25lkp@intel.com/T/#u

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Link: https://lore.kernel.org/r/1589875064-662-1-git-send-email-yong.wu@mediatek.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>