iommu/vt-d: fix wrong return value of dmar_table_init()

If dmar_table_init() fails to detect DMAR table on the first call,
it will return wrong result on following calls because it always
sets dmar_table_initialized no matter if succeeds or fails to
detect DMAR table.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>