iommu/amd: Return an error if vCPU affinity is set for non-vCPU IRTE

Return -EINVAL instead of success if amd_ir_set_vcpu_affinity() is
invoked without use_vapic; lying to KVM about whether or not the IRTE was
configured to post IRQs is all kinds of bad.

Fixes: d98de49a ("iommu/amd: Enable vAPIC interrupt remapping mode by default")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20250404193923.1413163-6-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>