iommu/vt-d: Silence an uninitialized variable warning

My static checker complains that "dma_alias" is uninitialized unless we
are dealing with a pci device.  This is true but harmless.  Anyway, we
can flip the condition around to silence the warning.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>