iommu/mediatek: Fix a build fail of m4u_type

The commit ("iommu/mediatek: Enlarge the validate PA range
for 4GB mode") introduce the following build error:

   drivers/iommu/mtk_iommu.c: In function 'mtk_iommu_hw_init':
>> drivers/iommu/mtk_iommu.c:536:30: error: 'const struct mtk_iommu_data'
 has no member named 'm4u_type'; did you mean 'm4u_dom'?
     if (data->enable_4GB && data->m4u_type != M4U_MT8173) {

This patch fix it, use "m4u_plat" instead of "m4u_type".

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>