KVM: arm/arm64: vgic-its: Fix vgic_its_restore_collection_table returned value

vgic_its_restore_cte returns +1 if the collection table entry
is valid and properly decoded. As a consequence, if the
collection table is fully filled with valid data that are
decoded without error, vgic_its_restore_collection_table()
returns +1. This is wrong.

Let's return 0 in that case.

Fixes: ea1ad53e (KVM: arm64: vgic-its: Collection table save/restore)
Signed-off-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>