iommu/mediatek: Only adjust code about register base

No functional change. Use "base" instead of the data->base. This is
avoid to touch too many lines in the next patches.

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Link: https://lore.kernel.org/r/20220503071427.2285-25-yong.wu@mediatek.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>