iommu/amd: fix type bug in flush code

write_file_bool() modifies 32 bits of data, so "amd_iommu_unmap_flush"
needs to be 32 bits as well or we'll corrupt memory.  Fortunately it
looks like the data is aligned with a gap after the declaration so this
is harmless in production.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>