virtio_vdpa: remove redundant check on desc

The boolean variable has_affinity is true when desc is non-null and
ops->set_vq_affinity is non-null. Hence the call to create_affinity_masks
does not need to check for desc being non-null is redundant when
has_affinity is true, so it can be removed as well as the now unused
default_affd.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Message-Id: <20241107132114.22188-1-colin.i.king@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>