iommu/vt-d: Suppress compiler warnings

suppress compiler warnings:
drivers/iommu/intel-iommu.c: In function ‘device_to_iommu’:
drivers/iommu/intel-iommu.c:673: warning: ‘segment’ may be used uninitialized in this function
drivers/iommu/intel-iommu.c: In function ‘get_domain_for_dev.clone.3’:
drivers/iommu/intel-iommu.c:2217: warning: ‘bridge_bus’ may be used uninitialized in this function
drivers/iommu/intel-iommu.c:2217: warning: ‘bridge_devfn’ may be used uninitialized in this function

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