KVM: s390: virtio_ccw: remove unused variable

Fix this warning:
drivers/s390/kvm/virtio_ccw.c: In function ‘virtio_ccw_int_handler’:
drivers/s390/kvm/virtio_ccw.c:891:24: warning: unused variable ‘drv’ [-Wunused-variable]
    struct virtio_driver *drv;

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>