iommu/vt-d: Clear the redundant assignment for domain->nid

Alloc_domain() will initialize domain->nid to -1. So the
initialization for domain->nid in md_domain_init() is redundant,
clear it.

Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>