iommu: Fix static checker warning in iommu_insert_device_resv_regions

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 6c65fb31 ("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>