iommu: Avoid unnecessary assignment of dev->iommu_fwspec

Move the assignment statement into if branch above, where it only
needs to be.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>