iommu/mediatek: Use correct fwspec in mtk_iommu_add_device()

The mtk_iommu_add_device() function keeps the fwspec in an
on-stack pointer and calls mtk_iommu_create_mapping(), which
might change its source, dev->iommu_fwspec. This causes the
on-stack pointer to be obsoleted and the device
initialization to fail. Update the on-stack fwspec pointer
after mtk_iommu_create_mapping() has been called.

Reported-by: Frank Wunderlich <frank-w@public-files.de>
Fixes: a9bf2eec

 ('iommu/mediatek: Use helper functions to access dev->iommu_fwspec')
Tested-by: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>