iommu: Fix static checker warning in iommu_insert_device_resv_regions

iommu: Fix static checker warning in iommu_insert_device_resv_regions

BugLink: http://bugs.launchpad.net/bugs/1671599
In case the device reserved region list is void, the returned value
of iommu_insert_device_resv_regions is uninitialized. Let's return 0
in that case.

This fixes commit 6c65fb318e8b ("iommu: iommu_get_group_resv_regions").

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
(cherry picked from commit a514a6e241f051dd8a4a00a456382dec0b1af21b)
Signed-off-by: dann frazier <dann.frazier@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>