iommu/mediatek: Remove unnecessary check in attach_device

This priv_data is set in the of_xlate. if of_xlate failed, it should
not enter attach_device. remove the unnecessary check.

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Link: https://lore.kernel.org/r/20210111111914.22211-32-yong.wu@mediatek.com
Signed-off-by: Will Deacon <will@kernel.org>