iommu/mediatek: Fix NULL pointer deference in mtk_iommu_device_group

In the Linux kernel, the following vulnerability has been resolved: iommu/mediatek: Fix NULL pointer deference in mtk_iommu_device_group Currently, mtk_iommu calls during probe iommu_device_register before the hw_list from driver data is initialized. Since iommu probing issue fix, it leads to NULL pointer dereference in mtk_iommu_device_group when hw_list is accessed with list_first_entry (not null safe). So, change the call order to ensure iommu_device_register is called after the driver data are initialized.