virtio scsi: fix unused variable warning

drivers/scsi/virtio_scsi.c: In function 'virtscsi_probe':
drivers/scsi/virtio_scsi.c:952:11: warning: unused variable 'host_prot' [-Wunused-variable]
  int err, host_prot;
           ^

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>