virtio-vdpa: Fix unchecked call to NULL set_vq_affinity

The referenced patch calls set_vq_affinity without checking if the op is
valid. This patch adds the check.

Fixes: 3dad5682 ("virtio-vdpa: Support interrupt affinity spreading mechanism")
Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Message-Id: <20230504135053.2283816-1-dtatulea@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Feng Liu <feliu@nvidia.com>