iommu/amd: Remove unused variable

The iommu variable in set_device_exclusion_range() us unused
now and causes a compiler warning. Remove it.

Fixes: 387caf0b ("iommu/amd: Treat per-device exclusion ranges as r/w unity-mapped regions")
Signed-off-by: Joerg Roedel <jroedel@suse.de>