iommu/vt-d: Remove unnecessary local variable initializations

A local variable initialization is a hint that the variable will be used in
an unusual way.  If the initialization is unnecessary, that hint becomes a
distraction.

Remove unnecessary initializations.  No functional change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>