iommu/amd: remove unused variable flush_addr

Variable flush_addr is being assigned but is never read; it
is redundant and can be removed. Cleans up the clang warning:

drivers/iommu/amd_iommu.c:2388:2: warning: Value stored to 'flush_addr'
is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>