iommu/mediatek: Fix NULL pointer deference in mtk_iommu_device_group

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.

Fixes: 9e3a2a643653 ("iommu/mediatek: Adapt sharing and non-sharing pgtable case")
Fixes: bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper probe path")
Reviewed-by: Yong Wu <yong.wu@mediatek.com>
Tested-by: Chen-Yu Tsai <wenst@chromium.org> # MT8183 Juniper, MT8186 Tentacruel
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Louis-Alexis Eyraud <louisalexis.eyraud@collabora.com>
Link: https://lore.kernel.org/r/20250403-fix-mtk-iommu-error-v2-1-fe8b18f8b0a8@collabora.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>