iommu/vt-d: Fix 64-bit accesses to 32-bit DMAR_GSTS_REG

[ Upstream commit fda3bec12d0979aae3f02ee645913d66fbc8a26e ]

This is a 32-bit register. Apparently harmless on real hardware, but
causing justified warnings in simulation.

Signed-off-by: CQ Tang <cq.tang@intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>