x86/intel-iommu: properly ack edu interrupt

Writing zero to EDU_REG_INTR_RAISE doesn't clear the status, because
the value is ORed in. The edu spec says that to clear the interrupt,
regardless of using MSI or not, one should write the value of the
interrupt status register to the interrupt acknowledge register from
the interrupt handler.

Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>