x86/amd-iommu: Rename iommu_flush_device

This function operates on a struct device, so give it a name
that represents that. As a side effect a new function is
introduced which operates on am iommu and a device-id. It
will be used again in a later patch.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>